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NBS SPECIAL PUBLICATION 



479 



U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards 




NATIONAL BUREAU OF STANDARDS 



The National Bureau of Standards^ was established by an act of Congress March 3, 1901. The Bureau's overall goal is to 
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" Located at Boulder, Colorado 80302. 



2 8 1977 



Corrosion and Metal Artifacts— 

)C\ A Dialogue Between Conservators and 
Archaeologists and Corrosion Scientists 



Edited by 

B. Floyd Brown 
Harry C. Burnett 
W. Thomas Chase 
Martha Goodway 
Jerome Kruger 
Marcel Pourbaix 



Sponsored by: 

National Bureau of Standards 

Smithsonian Institution 

American University 

Washington, Conservation Guild 

Belgian Center for Corrosion Study (CEBELCOR) 




U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary 

Dr. Sidney Harman, Under Secretary 

Jordan J. Baruch, Assistant Secretary for Science and Technology 

I NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director 

I ■ ■ 

Issued July 1977 



PREFACE 



In March, 1976, conservators of cultural property, archaeologists, curators, museum 
scientists, corrosion scientists, corrosion engineers, and metallurgists traveled from many 
countries to meet at Gaithersburg, Maryland, in the United States at the National Bureau of 
Standards. Our meeting was entitled "Corrosion and Metal Artifacts: A Dialogue Between 
Museum Conservators and Archaelogists and Corrosion Scientists." This volume is the formal 
report of the proceedings. 

This meeting in 1976 was the direct outgrowth of the first Rutherford John Gettens 
Memorial Seminar of the Washington Conservation Guild, held in 1975 at the Freer Gallery of 
Art on the same topics. The late Rutherford John Gettens was a pioneer in the study of 
ancient metals, especially the study of ancient bronzes and their corrosion. He set a high 
standard in a lifetime of work on the technical examination of artifacts. 

The success of the 1975 meeting was such that the Washington Conservation Guild joined 
Professor Marcel Pourbaix of the Centre Beige d'Etude de la Corrosion (CEBELCOR) in suggesting 
that a "Corrosion Week," one in a continuing series of Corrosion Weeks which have been held 
on both sides of the Atlantic for a number of years, be devoted to the same topic. We are 
especially grateful to Dr. Jerome Kruger for obtaining the generous support of the Bureau 
for this meeting and for the publication of these proceedings. 

The conservators, the scientists, and the archaeologists set the stage for our dialogue 
with lectures in which they gave their backgrounds and their points of view. The discussions 
these papers elicited have been included substantially as they occurred. Many of the ideas 
presented are set down for the first time here. Questions for the structured discussions 
had been collated by the program committee from questions submitted in advance of the meeting 
by the participants; the discussion of these questions is given in full, and we hope by 
reporting the discussion faithfully this report will be a substantial addition to the technical 
literature which has issued from the laboratories of the world's museums for many decades. 
Those who would see more of this literature should consult the Art and Archaeology Technical 
Abstracts, published twice a year by the International Institute for Conservation. 

We hope that as you read this volume of proceedings you will be carried beyond the 
methods of conservation and the techniques of the scientific laboratory, however interesting, 
to consider our fundamental interest in the objects. They represent the material heritage 
of mankind. These tangible things must be preserved and so our first preoccupation is with 
the conservation of these objects by the best means we can devise. Solving the problems of 
conservation is one step, an important step, toward a larger purpose. This larger purpose 
is the study of man through the things he has made. 

The study of human culture through material artifacts is gradually becoming recognized 
as a distinct discipline. It is a discipline to which the skill and insight of the conservator 
and the instrumentation of the scientist are indispensable, but it is a discipline which has 
its own structure, methodology, difficulties and rewards; one which demands serious commitment. 

Those who were fortunate enough to take part in the Dialogue are well aware of the 
splendid arrangements made for them by Ron Johnson and Paul Fleming of the Institute for 
Materials Research, assisted by Gloria Serig and others of the Institute Office, as well as 
Sara Torrence and the other members of the NBS conference staff. We appreciate the efforts 
of Ellen Ring and the staff of the Institute Text Editing Facility in preparing the copy, 
and especially those of Rosemary Maddock for preparing the illustrations, laying out the 
copy and coordinating the preparation (and the editors). 

This volume contains much material which has not been published before. We hope that 
the publication of so many useful ideas will prove stimulating to the old hands, and that 
this volume will be a helpful guide to the worker new to the study of artifacts. 



The Editors 



iv 



ABSTRACT 



This book is the formal report of the proceedings of the seminar on Corrosion and Metal 
Artifacts. It contains the tutorial lectures on the aspects of corrosion science and 
Engineering of relevance to conservators and archaeologists, background lectures which are 
addressed to corrosionists with activities and problems in the conservation of metallic 
artistic artifacts and the full discussion (attendee) of the structured questions distributed 
before the meeting. The report is well documented with illustrations. 



Key words: Archaeological finds, preservation of; conservation of metal artifacts; corrosion, 
inhibiting of; corrosion, treatment methods; metal artifacts, restoration of; patina, 
artifically produced; patina, natural. 



Disclaimer: Although the editors have reviewed all the papers and discussions contained in 
these proceedings, the views expressed are entirely the responsibility of the 
individual authors. 

In order to specify the procedures adequately, it has been necessary to identify 
commercial materials and equipment in these proceedings. In no case does such 
identification imply recommendation or endorsement by the National Bureau of 
Standards, nor does it imply that the material or equipment is necessarily the 
best available for the purpose. 



CONTENTS 

PAGE 



FOREWORD iii 

PREFACE iv 

ELECTROCHEMICAL CORROSION AND REDUCTION 

Marcel Pourbaix 1 

CORROSION PRODUCT CHARACTERIZATION 

N. A. Nielsen 17 

PRINCIPLES OF GASEOUS REDUCTION OF CORROSION PRODUCTS 

C. Ernest Birchenall and Russell A. Meussner 39 

SOME BRIEF REMARKS ON ELECTROCHEMICAL REDUCTION 

Jerome Kruger 59 

MEASURES FOR PREVENTING CORROSION OF METALS 

R. T. Foley 67 

A REVIEW OF THE HISTORY AND PRACTICE OF PATINATION 

Phoebe Dent Weil 77 

THE PRODUCTION OF ARTIFICIAL PATINATION ON COPPER 

D. C. Hemming 93 

BETA IRON OXIDE HYDROXIDE FORMATION IN LOCALIZED ACTIVE CORROSION 
OF IRON ARTIFACTS 

F. Zucchi , G. Morigi, and V. Bertolasi 103 

THE CURRENT STATUS OF THE TREATMENT OF CORRODED METAL ARTIFACTS 

R. M. Organ 107 

SOME CONSTRUCTIVE CORRODINGS 

Cyril Stanley Smith 143 

CONSERVATION OF RUSTY IRON OBJECTS BY HYDROGEN REDUCTION 

L. Barkman 155 

RESTORATION OF LARGE GILDED STATUES USING VARIOUS ELECTROCHEMICAL 
AND METALLURGICAL TECHNIQUES 

Fielding Ogburn, Elio Passaglia, Harry C. Burnett, 

Jerome Kruger, and Marion L. Picklesimer 167 

PROBLEMS OF RETRIEVAL AND RETENTION OF ARTIFACTS IN FIELD EXCAVATIONS 

W. Trousdale 179 

STRUCTURED QUESTIONS 191 

LIST OF ATTENDEES 241 



vii 



COLOR PLATE CAPTIONS 

a Roof constructed from copper sheet with artificial patina (p. 102). 

b Typical non-uniform mirror image patterns developed on adjacent copper 
sheets (p. 96). 

c Base silver cup from Enkomi in Cyprus, probably 14th century B.C. 
as excavated (p. 114). 

d Base silver object shown in figure c after cleaning (p. 114). 

e Cross-section from the haft of broad-axe of the type ch'i (p. 214). 

f Micro-section of chisels taken from Jericho (about 6000 B.C.) (p. 122). 

g Crystals never before observed in nature found on tin pannikin (p. 132). 

h A copper-tin alloy object showing typical bronze disease (p. 124). 

i Silver lyre from the Royal Graves at Ur (about 25 B.C.) (d. 120^. 

j Patina on bronze from the late Chou period (p. 207). 

k Bronze metal showing bronze disease (p. 124). 

1 Flake taken off a piece of bronze furniture from Nimrud (p. 111). 

m Cross-section of copper pit showing red CU2O and white CuCl beneath a 
mushroom of green malachite (p. 15). 

n A completely mineralized high tin bronze (p. 192). 

0 Three mirrors from the Man-ch'eng tombs (p. 192). 

p Detail of Japanese sword guard (tsuba) , 19th century (p. 151). 

q A bronze treated by an electrolytic method (p. 128). 

r Range of possible shades of natural patinas on copper (p. 93). 



vi ii 




d 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



ELECTROCHEMICAL CORROSION AND REDUCTION 



Marcel Pourbaix 

Managing Director - CEBELCOR 
Avenue Paul Heger, Grille 2 
1050 Bruxelles, Belgium 



The following text was adapted from tutorial lecture notes, prepared by Prof. Pourbaix 
expressly for the dialogue to accompany demonstration experiments. The figures are adapted 
from reference [1]^ (except fig. 14, which is taken from ref. [2]). References [3] and [4] 
give further tutorial material. The demonstrations were performed by M. Jean van Muylder 
(also of CEBELCOR), who also made the oral narration at the dialogue. 

Figure 1 indicates schematically some corrosion experiments demonstrated by M. van 
Muylder. In 1 to 12 (experiment a), an iron piano wire is immersed in water and in several 
solutions. In 1, distilled water corrodes iron in the presence of air, with the formation 




AISI 430 AISl 304 AlSl 316L 



Figure 1. Demonstration experiments. 



Figures in brackets indicate literature references at the end of this paper. 

1 



of brown rust (ferric oxides). In 2 and 3, the corrosion velocity is enhanced by chloride 
or by sulfuric acid. In 4, the corrosion may be decreased by a reducing substance (bisulfite); 
the rust then becomes black magnetite. In 5 and 13 (experiment b) , corrosion may be suppressed 
by caustic soda if air is present, but may be completely suppressed by chromate; but chromate 
may lead to severe pitting (7) if chloride is present and if chromate is not sufficiently 
concentrated ("dangerous inhibitor"). In 8 to 11, oxidants such as potassium permanganate 
and hydrogen peroxide may suppress any corrosion if their concentration is sufficiently 
high; on the other hand, they may very much intensify corrosion if their concentration is 
too low. In 12, it is seen that the Gaithersburg tap water is corrosive when stagnant; it 
is not corrosive when running. It is shown in 14 to 17 (experiment c) that the corrosion 
of iron is increased by contact with some metals, such as copper and platinum, but it is 
suppressed by contact with zinc or magnesium. In 18 to 21 (experiment d), an electric 
current is passed through two electrolysis cells, between two iron wires dipped in a 0.10 
molar solution of sodium bicarbonate. Both the two negative electrodes and one of the 
positive electrodes (in the right-hand cell) remain uncorroded, but the other positive 
electrode (in the left-hand cell) corrodes heavily, although the metals, the solution, and 
the intensity of the current are the same in both cells. We have thus, under similar condi- 
tions, anodic corrosion in the left and anodic protection in the right-hand cell. The only 
difference between the two cells is that there is, on the left one, a switch which does not 
exist on the right one, and which was closed for a few seconds at the beginning of the 
experiment. A device similar to that described above is shown in 22 to 25 (experiment e). 
An electric current is passed through the same two cells, between the same iron electrodes 
dipped into the same solutions; but instead of having placed a switch on the left one, we 
are passing there some oxygen around the negative electrode. It is observed that under 
these conditions, both the two positive electrodes and one of the negative electrodes (in 
the right-hand cell) remain uncorroded, but the other negative electrode (in the left-hand 
cell) corrodes. As was the case in the previous experiment, the metals, the solution, and 
the density of the current are the same in both cells. We have here, under similar circum- 
stances, cathodic -protection in the right cell, and oathodic corrosion in the left one. 



Air or Oxygei 



Electron Flux 



Figure 2. Production of a current 
by differential aeration (Evans 
experiment) . 



mA 

Potassium Chloride 
Solution 



Metal 




• Three-Way-Tap 



Porous 
Membrane 



Metal 



Figure 2 shows the famous historical experiment of differential aeration which was 
performed in 1923 by Ulick R. Evans. Two iron specimens connected through a galvanometer 
with center zero are placed in a solution containing some chloride and separated by a porous 
membrane. When air or oxygen is passed into the left hand compartment, electric current 
passes through the galvanometer, and it is evident that oxygen is promoting the corrosion of 
the iron in the right hand compartment, where it is absent. If the three-way cock is turned 
so that oxygen is passed into the right hand compartment instead of the left, the direction 



2 



of the current progressively changes and, once again, corrosion of the iron is promoted in 
the compartment where oxygen is absent. 

Such facts, as well as some others, sometimes funny ones, have often led to the opinion 
that corrosion phenomena are strange and often not understandable, and even do not obey the 
laws of thermodynamics. However, these facts may be easily explained if one considers that 
most of the reactions involved are not chemical, but rather electrochemical. 

Chemiaal reactions are reactions in which only chemical species are taking part, for 
example the chemical dissociation of liquid water into H+ and OH" ions: 

H2O = H"^ + OH" (1) 



or the chemical dissociation of water into gaseous hydrogen and oxygen: 



2H2O = 2H2 + O2 . (2) 



EleGtroahemiaal reactions are reactions in which not only chemical species, but also 
electric charges, i.e., negative electrons, e", are taking part; for example, the reduction 
of hydrogen ions to gaseous hydrogen: 

+ 4e" = 2H2 (3) 



and the oxidation of water to gaseous oxygen: 



2H2O =02+ 4H^ + 4e" . (4) 



The combination of these two electrochemical reactions leads to the overall chemical 
reaction of the dissociation of water (eq. (2) above) 



2H2O = 2H2 + O2 



which may thus be performed either chemically or electrochemically. 

Concerning the ahemioal reaction H2O + H"*" + OH" (1), it is very well known that, at 
25 "C, an aqueous solution is neutral as far as its acidity and its alkalinity are concerned 
when its activities in H+ and OH" are both equal to 10-''-°o gram ions per liter, i.e., when 
its pH is 7 (at 25 °C). 

For studying electrochemical reactions, it is convenient to use the concept of electrode 
potential, which may be measured as shown in figure 3. If a metal is in contact with an 
electrolyte (for example an aqueous solution), its electrode potential is the electric 
potential of this metal measured versus the electric potential of a reference electrode (on 
which a reversible_electrochemical reaction, such as H2 = 2H''' + 2e for a hydrogen electrode, 
or 2Hg = Hg2"*' + 2e" for a calomel electrode, is taking place under conditions of equilibrium), 
the solution in contact with the metal and the solution of the reference electrode being 
connected with an electrolytic siphon presenting no diffusion potential (for instance a gel 
of agar-agar saturated with potassium chloride). The value of this electrode potential is, 
in fact, a measure of the oxidizing or reducing power of the interface between the metal 
considered and the solution in contact with it, at an area close to the extremity of the 
siphon (salt bridge) . 



3 



Metal 



Figure 3. Measurement of the 
electrode potential. 




Solution 

H2O, O2, 

Fe(0H)2 

Fe304 

Fe203 

H2O3 



Reference Electrode 



The concepts of pH and of electrode potential may be represented as in figure 4, valid 
for 25 °C. Note the vertical line in this diagram at pH 7: along this line the solutions 
are neutral as far as their acidity and their alkalinity are concerned; on the left side of 
this line, where the pH is less than 7, the hydrogen ion concentration is higher than the 
hydroxyl ion concentration, and the solution is acid; on the right hand side of this line, 
where the pH is greater than 7, the hydroxyl ion concentration is higher than the hydrogen 
ion concentration, and the solution is alkaline. 





2.0 


1 1 1 1 
Oxidizng and Acidic Medwm 


—\ r 1 1 

Oxidizing and Alkalme Medium 






1.6 










1.2 










0.8 








> 


0.4 








X 
UJ 


0 


-^^^^^ 








-0.4 










-08 


Reducing and Acidc Medium 


Reducng and Alkolne Medium 






-12 










-16 


1 1 1 1 


1 1 1 1 





Figure 4. Acid, alkaline, oxidizing, 
and reducing media. 



8 K) 12 14 16 



pH 



4 



The oblique line of equation E = +0.813 - 0.0591 pH v (SHE) (volts versus the standard 
hydrogen electrode) indicates the conditions where the partial equilibrium pressure in 
gaseous hydrogen is twice the partial equilibrium pressure in gaseous oxygen: ^Poji 
along this oblique line, aqueous solutions may be considered neutral as far as reduction and 
oxidation are concerned. The conditions of absolute neutrality, as far as acidity and 
alkalinity, and oxidation and reduction are concerned, is thus represented by the point of 
intersection of these two lines, whose coordinates are pH = 7, E = -0.400 volts (SHE). The 
area of figure 4 is thus divided into four regions: top left, oxidizing and acidic media; 
top right, oxidizing and alkaline media; bottom right, reducing and alkaline media; and 
bottom left, reducing and acidic media. 

Along line a^ of figure 4, which represents the equilibrium conditions of reaction H2 = 
2H + 2e", and whose equation is E = 0.000 - 0.0591 pH volts (SHE), liquid water is, at 
25 °C, in thermodynamic equilibrium with gaseous hydrogen under one atmosphere pressure; 
below this line £, water may be reduced to hydrogen, but this may not occur above line a^. 
Along line b^, which represents the equilibrium conditions of reaction 2H2O = 02 + 4H+ + 4e' , 
and whose equation is E = +1.228 - 0.0591 pH volts (SHE), liquid water is, at 25 °C, in 
thermodynamic equilibrium with gaseous oxygen under one atmosphere pressure; above this line 
b^, water may be oxidized to oxygen, but this may not occur below line b^. Between these 
lines a and b^, water may be neither reduced nor oxidized and is thus thermodynamically 
stable under one atmosphere pressure. The area between lines a^ and b^ is thus the area of 
thermodynamio stability of liquid water at 25 °C and under one atmosphere pressure (fig. 5). 
And such areas of stability exist not only for liquid water but also for all the other 
species (solid, liquid, gaseous or dissolved) likely to occur in the presence of water. 



2 ,0 - 
1.6 - 



"I 1 1 1 1 1 

Liberation of Oxygen and Acidification 




Figure 5. Region of thermodynamic 
stability of water under a pressure 
of 1 atmosphere at 25 °C. 



Figure 6 shows a potential -pH diagram for the iron-water system. This figure shows 
that the oxidation of iron may lead to soluble products (green ferrous ions Fe^"*", yellow 
ferric ions Fe'*""'"'*", and green dihypo-ferric ions Fe02H') or to insoluble products (white 
ferrous hydroxide Fe(0H)2 unstable to black magnetite Fe30i+ and brown ferric oxide Fe203 
which may be variously hydrated and is the main constituent of rust). We shall consider, 
for fixing the idea, that iron is corroding in the presence of an iron-free solution where 
the quantity of solution which this solution may dissolve is greater than a given low value 
{e.g., 10"^ gram atom/liter, or 0.056 ppm) , and that iron may be rendered passive if it 



5 




becomes covered with a protective insoluble oxide or hydroxide {e.g., Fe203). Then the 
lines, which are drawn in figure 6 corresponding to a solubility of metal and its oxide 
equal to 10"^, delineate two areas where corrosion is possible (areas of corrosion), an area 
where corrosion is impossible because the metal is thermodynamically stable (area of immu- 
nity), and an area where passivation is possible (area of passivation). Figures 7a and 7b 
represent the theoretical conditions of corrosion, immunity, and passivation of iron, assuming 
that the insoluble oxides Fe203 and FesOi^ are sufficiently adherent and impermeable so that 
corrosion of the underlying metal is essentially stifled and that the metal is then "passive." 



(q) (b) 




Figure 7. Theoretical conditions of corrosion, immunity and passivation 
or iron: (a) assuming passivation by film of Fe203; and (b) assuming 
passivation by films of Fe203 and Fe30i+. 



6 



If the pH and the electrode potential of the experiments of figure 1 are measured, and 
the results of these measurements plotted in figure 8, with notation made of general corrosion 
( •), of local corrosion ( <» ) and of absence of corrosion ( o), one observes that the 
conditions under which there is effective corrosion or absence of corrosion are in good 
agreement with the theoretical predictions. Especially this figure shows that an oxidizing 
action either protects iron, or causes increased corrosion, depending on whether the parti- 
cular value of electrode potential of the metal falls within the area of passivation or not. 
Also the figure shows that the corrosion of iron in a degassed solution of caustic soda is 
due to an area of corrosion in alkaline solution free from oxidants. In experiment d, (fig. 
1) the effect of the switch has b^n to move down the electrode potential of the passive 
anode 21 to the corrosion value nj) . Also in experiment e, (fig. 1) the effect of bubbling 
oxygen has been to move the electrode potential of the immune cathode 24 to the corrosion 
value 22. 



X 
UJ 

o 
c 



■10 





1 




1 1 






02I 














>v oil 




- 


CORROSION 


•'° 9^ 


PASSIVATION 






4 2 iVVSf 19 

12 14 22^ 




IMMUNITY 


°I5 ig 

OI6 820 






1 


1 1 


CORROSION-^ ~ 

1 1 



6 8 
pH 



14 



Figure 8, Theoretical and experimental conditions of corrosion, 
• , and noncorrosion, o , of iron at 25 °C. 



Many other, somewhat funny, experiments may be explained by such diagrams; notably, 
referring to figure 1, (experiment f): behavior of iron in nitric acid solutions of different 
concentrations. At low concentrations, considerable corrosion with continuous evolution of 
nitrogen and of oxides of nitrogen; at high concentrations, no corrosion; at intermediate 
concentrations, alternate periods of corrosion with gas evolution and of no corrosion. 
Experiment g: behavior of carbon steel successively immersed in a solution of concentrated 
nitric acid and in a solution of copper sulfate; the steel becomes passive, but its passi- 
vity is destroyed by a blow from a glass rod. Experiment h: "stainless" steel in the 
presence of a solution of sodium chloride. A simple rubber band may cut chromium steel AISI 
430 and a chromium-nickel steel AISI 304 by crevice corrosion; AISI 316 steel, containing 
chromium, nickel, and molybdenum, is more resistant. 

As has just been shown, the potential-pH equilibrium diagram for iron may help to 
understand the corrosion behavior of iron in the presence of different substances. Simi- 
larly, potential -pH equilibrium diagrams for these different substances may yield useful 
information on the behavior of these substances; we shall consider how in the behavior of 
hydrogen peroxide in experiments 10 and 11 of figure 1, and in the behavior of potassium 
permanganate in experiments 8 and 9. 



7 



Figure 9 represents the conditions of equilibrium for the reduction of hydrogen per- 
oxide H2O2 (and its ion HO2) to water and for the oxidation of hydrogen peroxide according 
to the reactions: 

H2O2 + 2H'^ + 2e" = 2H2O (5) 
H2O2 = 02 + 2H'^ + 2e'. (6) 



In the area below the family of lines (2,3), hydrogen peroxide may be reduced to water 
according to reaction (4); in the area above the family of lines (4,5), hydrogen peroxide 
may be oxidized to oxygen according to reaction (6); in the area between these two families, 
both reactions are simultaneously possible. This corresponds to the decomposition of 
hydrogen peroxide by the overall chemical reaction 



2H2O2 ^ 2H2O + O2 



Concerning, in figure 9, point 10 (dilute peroxide solution) and point 11 (concentrated 
peroxide solution), it is evident that point 10, for which there is corrosion of iron, is in 
the area of reduction of hydrogen peroxide to water. Then, the reaction of hydrogen peroxide 
in dilute solution results from a combination of the corrosion reaction Fe -y Fe"*""*" + 2e" with 
the reaction H2O2 + 2H+ + 2e ^- 2H2O corresponding to the overall reaction Fe + H2O2 + 
2H+ ^ Fe++ + 2H2O. 

Point 11 (see figs. 8 and 9), for which there is no corrosion of iron, is in the area 
of double instability (or of chemical decomposition) of hydrogen peroxide; iron passivated 



20 

I 6 

12 

08 

_ 04 
> 

-04 
-0.8 

- 12 

-1.6 

-2 0 2 4 6 8 10 12 14 16 
PH 

Figure 9. Equilibrium potential-pH diagram for the system 
H2O2-H2O at 25 °C. 




8 



by the formation of a film of Fe203 there catalyzes the decomposition of hydrogen peroxide 
according to the reaction 2H2O2 2H2O + O2. This reaction accounts for the evolution of 
oxygen which is effectively observed on the surface of passive iron in experiment (11). 

Understanding the activating or passivating action of permanganate is based on the 
manganese-water equilibrium diagram (fig. 10). Comparing example 8 (dilute solution) and 
example 9 (concentrated solution), shown in this figure, it is observed that point 8, for 
which there is corrosion of iron, is in the area of stability of soluble Mn'*"'' ions; and that 
the point 9, for which there is no corrosion of iron, is in the area of stability of insoluble 
manganese dioxide Mn02. Thus the activating action of dilute permanganate solution results,^, 
from the combination of the corrosion reaction Fe ^ Fe''"+ + 2e~ with the reaction MnO^" + 8H + 
5e" -> Mn + 4H2O, corresponding to the overall reaction 2Mn04 + 5Fe + 16H+ ^ 2Mn'''+ + 
5Fe++ + 8H2O. 




Figure 10. Equilibrium potential -pH diagram for the system 
Mn-H20 at 25 °C. 

The passivating action of concentrated permanganate solutions is due to the combination 
of the passivation reaction 2Fe + 3H2O ^ Fe203 + 6H+ + 6e" with the reaction MnO^ + AH"*" + 
3e" ^ Mn02 + 2H2O, giving the overall reaction ZmO^ + 2Fe + 2H+ ^ 2Mn02 + Fe203 + H2O. 
Hence, the passive film is made up not only of iron oxide as is generally the case but of a 
mixture of iron oxide and manganese oxide. 

***** 



I am now coming to the corrosion behavior of artistic or architectural structures of 
carbon steels or of low alloy steels. Probably you all know that certain high strength low 
alloy steels HSLA of the ASTM type A-242, known in the USA mostly as Corten steels or Maraging 
steels, may sometimes resist atmospheric corrosion without being painted, due to the formation 
of a protective rust. 

The first building made of such steel was in Moline, Illinois. Other structures built 
with this type of steel include the Picasso sculpture in front of the Chicago Civic Center. 



9 



Some difficulties are often encountered when steels are used, notably because they do not 
resist corrosion by stagnant water and because it usually takes several years before the 
rust is really protective; in the meantime severe staining may occur which can be unesthetic, 
particularly where the staining occurs on adjoining concrete or stone. Several architects 
may sometimes overcome these difficulties as has been done in Castel Romano, Italy, where a 
conveniently located metallic grate allows the staining solutions to be drained away out of 
sight. 

Several problems are still to be solved before the scientific and technical aspects of 
these so called "weathering steels" may be considered as fully elucidated and mastered, 
mainly concerning an acceleration of the patina formation, which would avoid staining and 
its damaging consequences, and concerning the production of patinas of different colors. 
These are two problems we are working on in Brussels. 

Figure 11 represents the influence of pH on the electrode potential of iron in the 
absence and in the presence of oxygen. One sees that oxygen may, according to the concen- 
tration, promote corrosion (area 2a) or stop corrosion (area 2b), so that the problem of 
forming quickly a protective patina on a low alloy steel is, in fact, the problem of increasing 
quickly and in a stable manner the electrode potential of the steel in the presence of rain 
water of pH about 6 to about +450 to +550 mV (SHE), i.e., about +200 to +300 mV (SCE). We 
have been conducting extensive research work along these lines and the results seem to be 
satisfactory. It has already been possible to produce protective patinas within a few hours 
instead of a few years, thanks to some pretreatments , and to produce patinas of different 
colors. This work is presently funded within the frame of a joint program of the European 
Coal and Steel Community ESCC. 



1.6 



-2. 



— r~ 
10 



— r- 
12 



1.6 




14 



pH 



Figure 11. Conditions for immunity after general corrosion 
and for perfect passivity of in the absence of oxygen 
(line 1) and in the presence of oxygen (lines 2a and 2b). 



10 



Potential -pH equilibrium diagrams similar to the ones we have been showing for iron, 
hydrogen peroxide and manganese have been drawn for all the metals and metalloids, and 
notably for the metals which are of interest for metal artifacts: gold, silver, copper, tin, 
lead and aluminum. Figure 12 shows the diagram for the system copper-water. It may be seen 
that, in the presence of an atmosphere of absolutely pure air, practically free from both CO2 
and SO2, that is at pH about 6 and somewhat below the oxygen line b^, the stable form of cop- 
per is the black cupric oxide CuO (tenorite); copper is thus covered with a black patina. 




pH 



Figure 12. Potential-pH equilibrium diagram for the 
system copper-water at 25 °C. 



Diagrams relating to the system CU-CO2-H2O, such as those of figure 13, show that, if 
the amount of CO2 in an atmosphere exceeds about 0.04 percent, corresponding to a rainwater 
of pH about 5.8 containing about 10"^ mole CO2 per liter {i.e., about 0.4 ppm CO2), the 
patina may consist of green basic carbonate CuC03-Cu(0H)2 (malachite) together with black 
tenorite. According to figure 13a, green malachite is the stable form of copper in an 
aerated city water of pH about 8 containing about 44 ppm dissolved CO2 {i.e., about 10 
French degrees of temporary hardness), and this is why, when used for delivery of cold city 
water, copper pipes are often internally covered with a green deposit of malachite. 

Diagrams such as the one in figure 14 [4] relating to the system CU-SO3-H2O, show the 
patina formed on copper in an atmosphere polluted by SO2, may consist of black tenorite or 
of green basic copper sulfate CuC04-3Cu(0H)2 (brochantite) depending on the amount of SO2. 

The diagrams of figure 15, relating to the system CU-CI-H2O, show that, in the presence 
of an aerated and acid solution containing chloride, the stable form of copper is the basic 
copper chloride, gamma CuCl2-3Cu(0H)2 (paratacamite) , which is the copper mineral existing 
in Chile in huge quantities. But where no oxygen is present, the stable copper derivative 
is the white cuprous chloride CuCl (nantokite). This occurs, for instance, at the bottom of 



11 





Figure 13. Equilibrium potential-pH diagrams for the ternary system CU-CO2-H2O at 25 °C: 
(a) 10-3 M/L CO2 total dissolved (44 ppm); (b) lO'^ M/L CO2 total dissolved (440 ppm); 
(c) 10"! M/L CO2 total dissolved (4400 ppm); and (d) 1 M/L CO2 total dissolved (44000 ppm). 



12 




Figure 14. Potential-pH equilibrium diagram for the ternary system CU-SO3-H2O at 25 °C 
for 10"3-2'+ n,ole dissolved SO3 per liter (46 ppm), ref. [2]. 



13 




-2 0 2 4 6 8 10 12 14 16 -2 0 2 4 6 8 10 12 14 16 

pH pH 



Figure 15. Potential-pH equilibrium diagrams for the ternary system CU-CI-H2O at 25 °C: 
(a) 10-3 g-ion CL'/L, (35 ppm); (b) lO'^ g-ion Cl"/L, (3550 ppm); (c) lO"! g-ion 
Cl/L, (3550 ppm); and (d) 1 g-ion Cl/L, (35,500 ppm). 



14 



Figure 16. Copper pit in the presence of Brussels water. See color plate m. 
Cross section shows the presence of red CU2O and of white CuCl beneath a 
mushroom of green malachite. 



dangerous pits sometimes formed inside copper tubes used for delivering cold Brussels water, 
shown in figure 16. At the bottom of a cavity containing loose red cuprous oxide CU2O, 
metallic copper is in contact with CU2O and with white cuprous chloride CuCl . The solution 
existing inside this cavity is thus saturated altogether with these two substances and the 
equilibrium conditions of the system at the bottom of the pit are approximately represented 
in the diagram (valid for 10"^ g ion CI" per liter) by the point of intersection of these 
straight lines 12 (equilibrium CU/CU2O), 51 (equilibrium CU2O/CUCI ) and 55 (equilibrium 
CuCl/Cu). The two coordinates of this point are, respectively, pH = 3.5 and E = +270 mV 
(SHE) (or + 20 mV(SCE)) and the amount of dissolved copper and chloride in the solution are, 
respectively, about 234 ppm Cu and 270 ppm CI. As the reaction Cu = Cu"*"*" + 2e~, which may 
take place in this solution, is reversible, anytime the electrode potential really existing 
inside the cavity will be greater (more positive) than the equilibrium potential +20 mV 
(SCE), dissolution of copper according to Cu -> Cu''"''" + 2e~ will proceed in the cavity, and 
this means corrosion. On the contrary, anytime the electrode potential inside the cavity 
will be lower (more negative) than +20 mV (SCE), redeposition of metallic copper will occur 
according to Cu^^ + 2e ^ Cu, and this means stifling of the corrosion. There is usually a 
diffusion potential of about 80 to 150 mV between the solution inside the cavity and the 
bulk of the water; when measured outside the cavity, which is the usual practice, the criti- 
cal value of electrode potential above which copper pits will develop is thus about +100 to 
+170 mV (SCE). And this seems to be true, not only for the localized corrosion of copper, 
but also for the localized corrosion of copper alloys, such as bronzes, brasses and copper- 
nickel alloys. 

Thus, when copper and copper alloys are in contact with an aggressive water, the 
development of pits and of other localized corrosion essentially depends upon the value of 
the electrode potential of the metal in the presence of the water: any substance whose 
electrode potential in this water is higher (more positive) than +100 to +170 mV (SCE) 
(platinum, gold, carbon ...) may promote pitting; any substance whose electrode potential 
will be lower than +100 to +170 mV (SCE) (silver, tin ...) may not promote pitting; sub- 
stances which have a significantly lower electrode potential (zinc) may not only not pro- 
mote pitting, but also stifle previously existing pits. 

Thus, reduction of corrosion products formed inside copper pits may be promoted by any 
treatment which will depress the electrode potential inside pits below about +20 mV (SCE), 
and preferably lower than this. This may be achieved either by a reactive anode [e.g., 
zinc) or by an impressed current. Another way to stifle pits is to use corrosion inhibitors 
[e.g., benzotriazole) which might render the corrosion reaction irreversible. 

Due to lack of time, the essentials of electrochemical kinetics have not been considered 
in the present conference; needless to say, they should be considered with the greatest 
care. 



15 



References 



[1] Pourbaix, M. , Leaons en Corrosion Eleatroahimique , 2nd French Edition (CEBELCOR, 
Brussels, 1975). 

[2] Pourbaix, M., Some applications of potential-pH diagrams to the study of localized 
corrosion. Palladium Award Lecture, J. Elea. Soc. , 1 33 , No. 2 (1976). 

[3] Pourbaix, M. , Lectures on Eleotroohemioal Corrosion, translated from the French by J. 
A. S. Green, R. W. Staehle, ed. , foreword by Jerome Kruger (Plenum Press, New York, and 
CEBELCOR, Brussels, 1973). 

[4] Pourbaix, M. , and van Muylder, J., Laboratory Experiments for Lectures on Electro- 
chemical Corrosion (CEBELCOR E. 65, 1967). 



Discussion 

C. Peterson: Is the selection of material for anodes critical in attempting to impose 
cathodic protection on iron artifacts stored in an aqueous solution? 

M. Pourbaix: If one uses an outside source for the electrical current, one may use any 
metal for the anode as long as one calculates the electrode potential correctly. 

E. V. Sayre: If one has a copper object that is selectively corroding in a number of 
small pits over its surface, are there any practical methods for inhibiting the corrosion 
by locally reducing the electrode potentials within the pits? 

M. Pourbaix: There is a very simple method. If you place a copper rod so that it touches 
the bottom of the pit, you enforce into the bottom of the pit an electrode potential that 
is low enough to inhibit further dissolution. One may also use a zinc electrode in the 
vicinity of the pit but one must be careful that the current flow is between the zinc and 
the corroded area. As in the case of iron, that was just discussed, one may also apply an 
external source. Finally, another way to do this is with the use of inhibitors such as 
benzotriazole. 

G. M. Ugiansky: I was wondering about the Picasso sculpture in Chicago. I heard several 
years ago that they were having trouble getting the patina or rust finish on the underside 
of the sculpture. They were having to hose the underside to get it to discolor. Has that 
type of problem been solved? 

M. Pourbaix: I must confess that I have never made measurements on the Picasso monument. 
We think that to have a good patina you have to have a high temperature, so you need sun. 
Some research has been done for using CD. steel for the lower part of cars. According to 
my experience, the sun only comes to this part when the wheels of the car are turned up. 
So I do not think that this is a good way to use these steels. So, too, at the bottom of 
the Picasso monument the temperature has probably not been high enough. But I have no 
opinion, as I only saw the structure at the beginning, so I don't know how it is now. 

I may say this, we have had great success with these steels and I am very much in 
favor of promoting these steels. There have been, however, many failures because people 
think that you can use these steels just like other steels. This is not true. These 
steels have a bright future but one should realize that it is a special material that has 
its own character. 

Unidentified: I would like to correct one impression. The Picasso monument never had a 
problem. It weathered very nicely. There was, what is called the "Abraham Lincoln Oasis," 
a restaurant over a highway on the south side of Chicago. The architect wanted to hose it 
down daily in order to develop the color in time for a dedication. 



16 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts — A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



CORROSION PRODUCT CHARACTERIZATION 



N. A. Nielsen 

Engineering Technology Laboratory 
Engineering Department 
E. I. du Pont de Nemours & Co., Inc. 
Wilmington, Delaware 19898 



The world-wide costs of corrosion, direct and indirect, are enormous, measurable in 
many billions of dollars annually. Corrosion scientists and engineers are professionally 
involved in a continuing struggle to reduce industrial corrosion of metallic materials of 
construction and to preserve metals^ from the many forms of corrosion attack to which they 
are susceptible. While much of their work involves materials such as steel, stainless 
steels, nickel-base and high alloy materials, etc., for which the conservator has little 
concern, the techniques employed in corrosion research and surface characterization and 
many of the corrosion phenomena studied can find direct application in the study and 
preservation of museum material. 

The examination of corrosion products is particularly important, because they are 
present on all metals. Gold is the single exception, thermodynamics indicating that gold 
oxide will not form at room temperature. Oxidic corrosion products exist in a variety of 
states. They can provide corrosion protection as thin, invisible barrier films on stain- 
less steels, chromium, aluminum, titanium, and other "corrosion-resistant" metals. They 
can also form unhindered, entirely consuming a metal as it reverts back to an ore or to a 
nonmetallic state. Much ancient iron and bronze has been found in this condition, the 
artifacts totally converted to oxide. 

It is, of course, obvious to the corrosion scientist that conservators and archae- 
ologists have a much narrower spectrum of interests in metals and their corrosion products. 
The number of metals in use prior to the 19th Century was relatively small and corrosion 
occurred in environments, air, soil and water (including marine), which were relatively 
pure by our present standards. 

In an early (1808) essay on conservation of medals, John Pinkerton [1]^ wrote: 

Nothing contributes so much to the oonservation of brass or copper coins as 
that fine rust, appearing like varnish, which their lying in a particular soil 
occasions. Gold admits no rust but iron mould, when lying in a soil impregnated 
with iron. Silver takes many kinds; hut chiefly green and red, which yield to 
vinegar. For in gold and silver the rust is prejudicial and to be removed; 
whereas in brass and copper it is preservative and ornamental. 

This statement is an echo of the ancients who recognized that heavy corrosion product 
layers on copper alloys had the dual function or circumstance of being protective as well 
as decorative. There was a Greek phrase^ which termed patina "the flower of brass." 

The aesthetic appreciation of a fine patina or a "noble patina," as Gettens [2] spoke 
of it, remains with us today; but perhaps less so with the corrosion scientist who finds 
in these corrosion product layers the opportunity to study and analyze a "corrosion test 



The term metals is used here to include all alloys. 

2 

Figures in brackets indicate literature references at the end of this paper. 

3av0OS xC'^kOu 



17 



specimen" that has been exposed for centuries or even tnillenia. It is this time factor 
which cannot be duplicated in any accelerated laboratory corrosion testing that makes the 
study of ancient metals so fascinating. It is somewhat analogous to the comparative study 
(for the physical metallurgist) of terrestrial steel with meteoritic iron-nickel alloys 
whose cooling rate is measurable in millions of years (and here also the study of oxidation 
and corrosion products is being undertaken). 

In figure 1 is shown a scheme which one investigator, J. B. Cotton [3], devised 20 
years ago for the examination of corroded metal specimens. This scheme is perfectly valid 
and usable today. But new developments in instrumental techniques for surface characteri- 
zation have, in effect, made obsolete the employment of destructive, wet chemical methods 
of analysis. Corrosion researchers can easily, quickly, and nondestructi vely obtain 
compositional data on metals and alloys and their environmental reaction (corrosion) 
products. 



Preliminary 
assessment by 
binocular 
examination 




METALLURGICAL EXAMINATION 
FOR CHECK ON STRUCTURE OF 
METAL 

SPECTROCHEMICAL ANALYSIS & 
WET CHEMICAL ANALYSIS FOR 
CHECK ON COMPOSITION OF METAL 

MICROCHEMICAL EXAMINATION OF 
CORROSION PRODUCTS FOR 
COMPOSITION 

ELECTROCHEMICAL TESTS, 
e.g., ELECTRICAL POTENTIALS 

X-RAY EXAMINATION FOR 
ADDITIONAL INFORMATION ON 
NATURE OF CORROSION PRODUCTS 
AND STRUCTURE OF METAL 

ELECTRON DIFFRACTION EXAMINA 
TION FOR IDENTIFICATION OF 
SURFACE FILMS, ETC. 

TESTING OF PHYSICAL AND 
MECHANICAL PROPERTIES OF 
METAL 




Assembly & 
consideration 
of evidence & 
information 



TECHNICAL 
REPORT 



Figure 1. Scheme for laboratory examination of corroded specimens. 



More recently (in 1973), the National Association of Corrosion Engineers issued 
Standard (RP-01-73), Recommended Practice - Collection and Identification of Corrosion 
Products [4]. It provides guidelines in the methods and techniques of collecting samples 
of corrosion products and describes some procedures which can be used to analyze and 
identify the corrosion products. In intended engineering applications, the information is 
used to resolve a corrosion problem by identifying: (a) the nature and type of attack, 
(b) the metal or metal phase that is attacked in the alloy, and (c) the environmental 
conditions that contributed to the corrosion. Obviously, the conservator is interested in 
similar answers relevant to corroded and oxidized artifacts. In Section 5 on Analysis and 
Identification Procedures, the Standard covers the following eleven techniques: 

• Microscopy (optical, electron and scanning electron) 

• X-ray diffraction 

• X-ray emission spectrography 

• Electron probe microanalysis 

• Spark source mass spectrometry 

• Optical emission spectroscopy 

• Infrared spectroscopy 



18 



• Ultraviolet and visible spectroscopy 

• Flame spectroscopy 

• Differential thermal analysis 

• Wet chemical analysis. 

A check list for collecting and analyzing corrosion products is included as well as a 
bibliography on analysis and identification techniques. Altogether, this Standard is a 
very useful and valuable publication. 

Diagnosis of the causes of corrosion has been greatly facilitated as has the develop- 
ment of new information on corrosion mechanisms. A discipline of surface science has in 
fact come into being in the last 20 years because the advent of ultrahigh vacuum techniques 
led to a "panoply of electron, ion and x-ray spectroscopic techniques capable of char- 
acterizing solid surfaces" [5]. Several of these new tools for surface analysis are 
summarized below. 

1. Augev' Eleotron Spectroscopy (AES) employs a spectrometer which measures the 
energies of secondary electrons, "Auger electrons," ejected from the specimen surface as a 
result of electron bombardment. The Auger electrons originate in the top 2 to 10 mono- 
layers of the surface and have energies characteristic of their parent atoms. The am- 
plitude of an Auger peak is essentially proportional to the number of the atoms of the 
specific element present in the area of the specimen surface being analyzed (by a 1 pm 
diameter beam) . 

A further development in AES has been made in very recent years which employs a 
scanning beam as the electron probe. The Scanning Auger Microprobe (SAM) provides Auger 
images which show elemental distribution over a few square microns in a depth of 10 A more 
or less. It is an extremely high sensitivity technique and can identify elements present 
in quantities down to 0.1 percent equivalent monolayer. 

It should be noted that AES (and SAM) are analogous to the much older electron micro- 
probe technique in which wavelength dispersive x-ray fluorescence is employed to obtain an 
elemental analysis of about a 1 cubic micrometer volume element of a specimen surface. 
The AES technique is of course much more sensitive to chemical species located in the top 
several atomic layers of the surface. Use of sputter-etching (beam of inert gas ions) to 
clean surfaces and to permit analysis in depth is essential if unambiguous data are to be 
obtained. 

2. Eleotron Spectroscopy for Chemical Analysis (ESCA) utilizes x-ray photons to 
ionize inner core electrons whose ejection energy corresponds to that of the photon minus 
the electron binding energy. The spectrum of ejected photoelectrons is used to identify 
the elements in the surface. It can also provide additional information on the chemical 
bonding states present in the surface. It is again a technique highly sensitive to 
surface composition (in the top 2 to 10 monolayers). An area of about 1 mm is irradiated 
with an x-ray beam. Since the x-ray beam cannot be used in a scanning mode, the ESCA 
technique does not permit spatial resolution or element mapping, as do the electron beam 
spectroscopies. 

3. Secondary Ion Mass Spectrometry (SIMS) represents a technique developed within the 
last several years in which an ion beam is used to probe the specimen surface. Secondary 
ions sputtered and eroded from the surface are collected and analyzed with a mass spectro- 
meter. The SIMS technique has very high sensitivity; its depth of analysis can be the 
equivalent of 1 to 2 monolayers. It can detect hydrogen and distinguish isotopes. For 
certain samples, SIMS can provide very useful information on chemical bonding when charac- 
teristic fragment ions from known compounds or polymers can be observed. 

4. Ion Scattering Spectrometry (ISS) examines the energies of the reflected ions (with 
the same beam that is used in SIMS). These energies are a function of the scattering angle 
and the masses of the incident ion and the ion from which it recoils. The ion beam diameter 
is 100 ym for both SIMS and ISS, but the latter spectroscopy provides an energy spectrum in 
which there is only one peak for each surface element. While this can make the interpre- 
tation of the spectra easier, two major problems are peak overlap in a chemically-complex 
surface and the inability of the technique to provide any information on chemical bonding. 



19 



5. Laser Miaroprobe Spectrometry (LMS) employs a combination of mass spectrometer and 
pulsed laser for pyrolysis studies. The technique is rapid and reproducible. The laser 
beam diameter can be as fine as 20 ym; the pulse duration is 200 y and a heating rate of 
about 10^° °C/s can be achieved. The technique is sensitive to inhomogeneities in solids 
(which can be sighted through a microscope on which the laser is mounted) and composition 
vs. depth profiles may be obtained by pulsing the same spot. It has been found that a 
0.85 J pulse will vaporize 1.87 x 10"^ gram of carbon [6]. 

(Reference at this point should be made to optical laser microprobe in use in the 
Museum of Fine Arts Research Laboratory for chemical analyses [7]. This system provides 
emission spectrographic analysis of material vaporized from a crater 50 to 80 ym in 
diameter and 80 to 100 ym deep.) 

All of these techniques have their strengths and weaknesses. While they are both 
competitive and complementary in their characteristics, no single technique can completely 
characterize a surface (or a corrosion product). It is necessary to select the one 
technique or combination of several techniques which will provide adequate information on 
the specific problem of the moment. 

Three major references pertinent to choosing analytical techniques for metallurgical 
(artifact) analysis are: Arohaeologioal Chemistry, C. W. Beck, Editor [8]; Kruger and 
Frankenthal 's chapter on Oxidation and Corrosion, in Vol. IV of Techniques in Metals 
Research, R. F. Bunshah, Editor [9]; and Authenticity in Art by Stuart J. Fleming [lOj. 
These references present a comprehensive picture of the use of analytical techniques by 
archaeologists, conservators and corrosion scientists in metallurgical analyses and 
corrosion product characterization. 



X-ray Diffraction 

The importance of x-ray diffraction techniques and analysis remains as important as 
ever in the structural identification of crystalline compounds and metallic lattices. In 
his 1956 article. Cotton [3] discussed how a corrosion product should be carefully removed 
for microchemical and x-ray examination with precautions being taken to avoid any inclusion 
of basis metal. The x-ray diffraction examination provides the possibility of determining 
what the compound species is whose composition is known from chemical analysis. 

Vaughan [11] has pointed out the importance of combined use of research tools in the 
analysis of corrosion products. Phase analysis by x-ray or electron diffraction cannot be 
overemphasized as a necessary procedure in corrosion research and corrosion product 
characterization. 

Kruger and Frankenthal [9] also provide a good discussion of the identification and 
measurement of the amount of corrosion products by the x-ray diffraction powder method. 
Barker has stated that it has been the "experience at the British Museum Research Laboratory 
that a very good all-round solution to the problems arising from a very wide range of 
archaeological material is provided by a combination of x-ray diffraction and spectro- 
chemical techniques" [12]. Barker points out very rightly that mixtures of crystalline 
compounds can provide patterns that are too complex to resolve satisfactorily and that 
sampling is a very important part of both x-ray and spectrographic analysis. It is wise 
to use a binocular microscope to separate obvious phases in a complex mixture or corrosion 
product so that each can be analyzed separately. On valuable objects, it is usually 
possible to do this using microtools and careful technique such that no visible damage 
will be done to the object. By combining a series of samplings from a corroded or patinated 
artifact, a qualitative analysis and a positive structural identification of all of the 
major constituents can be made. In most cases, the knowledge of composition and of 
structure of the object can determine authenticity or answer a problem in conservation. 



Scanning Electron Microscopy (SEM) and Energy-Dispersive 
X-ray Analysis (EDXA) 

It has been the writer's experience that structure-composition data on corrosion 
products is very conveniently attainable by applying standard metal! ographic techniques, 



20 



including x-ray diffraction, using a scanning electron microscope equipped with an accessory 
system for energy-dispersive x-ray analysis. 

The SEM did not become a conmercial instrument until 1965, but it has received an 
extraordinarily favorable acceptance as a tool for metallurgical research. It has become 
(perhaps more so than any other analytical instrument utilizing an electron beam probe) an 
extremely versatile apparatus amenable to a total systems approach to instrumentation. 
Its potential application to the study of archaeological material was recognized in 1970 
by Brothwell [13] who considered this an important new field. 

Figure 2 shows in simple fashion the variety of signals generated in an SEM when a 
finely focused beam (100 A diameter) of electrons is scanned over a specimen surface. 
Each of the electron signals can be used to "image" the sample so that it can be observed 
visually on a cathode-ray tube (CRT), photographed, or recorded on video tape. Resolution 
with secondary electrons is commonly better than 100 A with the newer SEMs. The use of 
backscattered electrons is desirable when there are compositional differences in the area 
being imaged. In this case, contrast in the topographic image is enhanced by an atomic 
number effect. 



INCIDENT 
ELECTRON 
BEAM 




Figure 2. Signals generated in the scanning electron microscope. 



With x-ray spectrometers, either energy-dispersive (ED) or wavelength-dispersive 
(WD), attached to the SEM, the x-ray distribution image becomes a useful method of pre- 
senting x-ray analytical information. In practice, as the electron beam scans the sur- 
face, the spectrometer is set at the particular element of interest. Whenever, at that 
energy or wavelength, an x-ray is detected, a dot is brightened at the corresponding point 
on the display CRT of the SEM. The image that results corresponds spatially to the other 
(electron) images of the surface but with the density of dots in each area related to 
elemental abundance. It should be pointed out, however, that the x-ray distribution image 
gives only very qualitative information on element concentrations and cannot be used when 
elements are present in low concentration in a surface. More commonly, a standard spectral 
analysis will be run, requiring no more than 5 minutes by energy-dispersive x-ray analysis 
(EDXA), and each peak on the energy spectrum separately identified for the element and the 
specific x-ray emission line producing it. It is, of course, also possible to get inte- 
grated counts of all the x-ray emissions in a sample, to photograph the data on a display 
CRT or to get a computer printout of the tabulated data. Hanson at the Winterthur Museum 
has carried out pioneering work on the quantitative elemental analysis of art objects by 
energy-dispersive x-ray fluorescence spectroscopy [14]. 



21 



On the question of spectral resolution, the wavelength-dispersive (WD) method is 
clearly superior. For example, the sulfur K and lead M lines at 2.31 and 2.34 keV, 
respectively, cannot be resolved by EDXA. It would seem that an ideal combination system 
would be to have an SEM with both types of spectrometers. The WD spectrometer would be 
used for detailed quantitative work where its high resolution, high peak-to-background 
ratio and ability to detect light elements (from Be, atomic number 4 upward; the ED system 
starts at Na, atomic number 11) would provide better performance. This technique, scanning 
electron probe microanalysis, is discussed by Heinrich [15]. 



Corrosion Products on Copper and Its Alloys 

In April 1959, the National Association of Corrosion Engineers (NACE) issued a 
Technical Committee Report, Publication 59-13 on "Identification of Corrosion Products on 
Copper and Copper Alloys" [16]. This report should be of interest to conservators and 
archaeologists since it gives a description of microchemical analytical methods by which 
both soluble and insoluble constituents can be identified. Constituents discussed in 
detail include oxides, chlorides, sulfates, sulfides, carbonates, silica, calcium, magnesium, 
sodium, ammonia, and metal constituents of the material being examined. Also a method for 
spectrographic examination of corrosion products is considered briefly. 

In the natural patination of unalloyed (commercial) copper, Schmidt [17] has shown 
that neither the type of copper nor its hardness or surface is of significant importance. 
What is important is the total time of exposure to water, its corrosivity and the in- 
clination of test specimens. A patina develops more rapidly on inclined surfaces (compared 
to vertical surfaces) which dry more slowly and are also subjected to additional attack by 
dew. Seven years of exposure of test specimens to a mill industrial atmosphere about 8 
miles west of Copenhagen and 3 miles from the shore resulted in all inclined specimens 
showing a bluish-green patination. The vertically exposed specimens showed only faint 
signs of patination and were all dark brown in color, probably because of a mixed oxide- 
sulfide tarnish layer. 

Analyses of the patina on the inclined specimens corresponded to a composition of 
CuCOit • 6 Cu(0H)2. (No x-ray diffraction data were reported.) Schmidt pointed out that 
the patina on old (30 years or more) copper roofs in Copenhagen corresponds to the mineral 
brochantite, CuSOi, • 3 Cu(0H)2. He commented that measurements of rain water collected in 
1944-1945 showed a nearly neutral pH (6-8) whereas in 1965 the pH of rain water was 4-5. 
The presence of SO2 in the atmosphere there, as in many places around the world, is 
causing much earlier patination of copper roofs. 

The writer has examined the patination of a copper roof of a 70-year old water tower 
in Wilmington, Delaware. By x-ray diffraction, the corrosion product layer was found to 
consist of brochantite, antlerite (CuSOij • 2 Cu(0H2), and copper oxide (CuO). In this 
case, there were 50 x-ray diffraction lines and all were identifiable with known compounds. 

Problems, however, can arise when corrosion products on copper alloys, particularly 
ancient alloys, are analyzed by x-ray diffraction. The patterns can be even more complex, 
as was found when a problematical Hasanlu bronze (3rd millenium B.C.) was examined. This 
artifact had been almost entirely converted to oxides and nonmetal 1 ics . A typical polished 
core section photographed with polarized light is shown in figure 3. The same field, at 
the same magnification, 150X, is again shown in figure 4 where secondary electron (SE) and 
backscattered electron (BSE) images can be compared, and three elemental distribution maps 
are included. The latter identify the spheroidal inclusion as being lead-rich (it is 
actually PbCOs) and show smaller angular inclusions to be iron-rich phases. 



22 



Figure 3. Hasanlu Bronze, 3rd Millenium B.C. (Polished core 
section of oxidized artifact, photographed with polarized 
light.) 




Figure 4. Oxidized Hasanlu bronze: 

(a) secondary electron image, 

(b) CuKcx x-ray image, 
(see page 24). 



23 




Because of the structural and compositional heterogeneity of this artifact, originally 
a leaded-tin bronze, the oxide scale was dissected and separated into separate color 
fractions which were then examined by x-ray diffraction. Ten different crystalline 
species were identified: 

malachite, Cu(0H)2 • CuCOj 
atacamite, Cu(0H)2 • Cu(OH, CI) 
paratacamite, Cu(0H)2 • CuCl 
calumetite, Cu(OH, Cl)2 • 2 H2O 
cuprite, CU2O 
tenorite, CuO 
lead 

lead carbonate, PbCOa 
copper-tin alloy 
magnetite, FesO^. 



A brown and white phase(s) was not identifiable. 



24 



Understanding of the complexities of corrosion product formation in ancient copper 
alloys is facilitated when the enthalpies of formation of the oxides and hydroxides of 
copper and tin (and zinc, in the case of antique brasses) are considered; Werner [18] has 
shown (table 1 below). 



Table 1. Enthalpies of formation. 



Enthalphy of formation 
Compound Kcal/mol 



CU20 


- 39. 


.8 


CuO 


- 37. 


,1 


ZnO 


- 83. 


,2 


Sn02 


-138. 


,1 


Cu(0H)2 


-106. 


,7 


Zn(0H)2 


-153. 


.5 


Sn(0H)2 


-270. 


,5 



From these data, it is predictable that tin- (and zinc-) rich phases in copper alloys will 
oxidize preferentially. Thus, in two-phase bronzes, the 6-phase, which is richer in tin 
than the a-phase, is more susceptible to oxidation (corrosion). Photomicrographs illustrate 
this not only for the copper-tin system, but show that the tin-rich phase in old pewter 
and Britannia metal also is less resistant to oxidation than lead-and antimony-rich phases. 
For example, an as-polished cross section of a Mesopotamian bronze shaft-hole axe** from 
the 3rd millenium B.C. (10 percent tin) is shown in figure 5. At least 50 percent of the 
original metal volume has been transformed interdendritical ly to oxide, which nucleated on 
tin-rich interfaces and converted the 6-phase preferentially to the a-phase. Where slip 
traces or deformation bands are present in the solid solution a-phase and have also under- 
gone preferential oxidation, it is assumed that segregation of tin atoms to these regions 
was responsible. 




Figure 5. Interdendritic oxidation in bronze Mesopotamian 
Axe, 3rd Millenium B.C. 



"^University of Pennsylvania Museum Number 31-71-186 (U. 14238). 



25 



Corrosion and oxidation occurring over a long-term time span also caused a sharply 
segregated distribution of copper and tin corrosion products (fig. 6). It appeared that 
dealloying of tin ( "destannif ication" ) had taken place. Tin will ionize and oxidize 
preferentially leaving behind a copper-enriched matrix. There was visible evidence of 
metallic copper in the corrosion product layers. 




Figure 6. Corrosion product layers in Hasanlu Axe. 



(The shaft-hole axe whose oxidation-embrittled microstructure is shown here also 
provided the opportunity for examination of mineralized wood structure present from the 
original hafting. The photomicrograph of figure 7 shows the cross-sectional structure of 
the wood remaining in the axe socket. No identification of the wood has been made.) 




Figure 7. Mineralized wood in Hasanlu Axe socket. 



26 



This specimen also showed the presence of transgranular stress-corrosion cracks (fig. 
8). At some time in its 4000+ year history, sufficiently high tensile stress was developed 
by the wedging action of oxidation/corrosion products formed under constraint to trigger 
crack propagation in whatever aggressive medium (presumably wet soil) the axe was then 
present and corroding. It is thought that this specimen represents the oldest known 
example of stress-corrosion cracking found in any alloy. 




Figure 8. Stress-corrosion cracks in Hasanlu Axe. 




Figure 9. Microstructure in bronze Amlash Arrowhead: (a) edge structure, (b) center structure. 



27 



In figure 9 are microstructures present in a bronze arrowhead (Amlash, 8th to 15th 
centuries, B.C.)- Here it is evident that the structure is one of a leaded, alpha bronze 
alloy which has undergone intergranular oxidation. The annealing following forging of the 
arrowhead did not completely obliterate the compositional heterogeneity of the original 
casting. Metal lographic etching (with an NH1+OH-H2O-H2O2 ) system brought out the primary 
dendritic system of the bronze. 



Deal loying 

Literature on the phenomena of deal loying, the corrosion process whereby one constit- 
uent of an alloy is removed preferentially from the alloy leaving an altered residual 
structure, has been reviewed by Verink and Heidersback [19]. 

Loss of tin from bronze has already been mentioned. However, loss of zinc from brass 
(dezincif ication) is the form of dealloying most commonly encountered. Alpha brass (70 




28 



Cu-30 Zn) is especially prone to this type of corrosion attack which results in the 
development of a weak porous copper sponge, either as a "plug" or a continuous layer in 
the brass surface. The cited reference [19] discusses recent work on mechanisms and shows 
how the application of potential/pH (Pourbaix) diagrams has led to new insights. 

Examination (by the writer) of brass spoons found in an historical archaeological dig 
(see next section on Corrosion of Pewter) showed that all pieces had suffered some de- 
zincif ication from long-term contact with wet soil and ground water. Figure 10 is included 
to show the nature of the phenomenon. Note that a "plug" of copper has filled in the 
dezincified region in the brass surface. In the extreme case of alloy failure by this 
mechanism, dezincif ication proceeds to the point where a cross section of brass can no 
longer support the load or applied stress upon it. 



Corrosion of Pewter and Britannia Metal 

The Archaeological Society of Delaware in its excavations at the Caleb Pusey home in 
Upland, Pennsylvania, found considerable artifactual material which provided needed informa- 
tion for subsequent restoration of the house (a small, one-and-a-half story cottage built 
in the late 1600's continuously occupied until the middle of the present century. It was 
known also as the "Billy Penn House." Penn made many visits there to see Caleb Pusey who 
was manager and agent for Penn's mill at Upland). Included in the material found were over 
two hundred coins as well as many spoons and spoon fragments of pewter [20]. The latter 
were analyzed by J. H. Carlson (Winterthur Museum) using energy-dispersive x-ray analysis 
who reported that they fell into three categories: 

(1) Three heavily corroded pieces of good quality pewter with tin levels greater than 
90 percent. 

(2) A large group of objects and fragments were classified as good to very poor 
pewter with tin contents ranging from about 80 percent down to less than 20 
percent. Many of the poor quality pieces were probably made by the residents of 
the house at the time, or by a local country pewterer. 

(3) A third group of so-called pewter objects and fragments are actually Britannia, 
containing 85 to 88 percent tin, 0 to 2 percent copper, 0 to 3 percent lead, and 
8 to 14 percent antimony. 

For metal lographic examination of pewter spoon fragments, it was convenient to cut, mount 
and polish a cross-sectional slice from incomplete handles or broken bowls. For example, 
figure 11 is a cross section of a heavily corroded spoon handle, a pewter containing 24 
percent lead, examined in the scanning electron microscope. The four images of the same 
area compare the secondary electron image with the backscattered electron image which shows 
greater contrast because the BSE mode emphasizes the heaviness of the atoms in the surface, 
i.e., responds to atomic number and a lead-rich phase is present. Figure 11 also illus- 
trates the usefulness of element x-ray mapping and shows that the lead-rich regions have 
resisted corrosion better than the tin-rich matrix phase. 



29 



(a) 



(b) 




(c) (d) 



Figure 11. Corrosion/oxidation of pewter (24 percent Pb): 

(a) secondary electron image, 

(b) backscattered electron image, 

(c) PbMa x-ray image, 

(d) SnLa x-ray image. 



Similar examination of a second spoon fragment identified as Britannia metal from its 
high antimony content, 14 percent, is shown in figure 12. This specific alloy is essen- 
tially a binary tin-antimony system with no copper and no lead. The cuboids, which are 
deficient in tin and show antimony segregation are, in this case, the corrosion-resistant 
phase. 



30 




(c) (d) 



Figure 12. Corrosion/oxidation of Britannia Metal (14 percent Sb): 

(a) secondary electron image, 

(b) backscattered electron image, 

(c) SnLa x-ray image, 

(d) SbLa x-ray image. 



Figure 13 is a micrograph (SEM using backscattered electrons) of a high-quality Bri- 
tannia metal plate (cross section). A heavy corrosion/oxidation layer exists on the 
bottom of the plate. This is not an archaeological find but represents what happened to 
the tin alloy in normal household usage over many years. Only the top surface of the 
dinner plate was ever cleaned and polished. The corrosion product is mixed Sn0-Sn02 (by 
x-ray diffraction) and has an onionskin structure. Intergranular corrosion attack has 
occurred in local areas ahead of the front of general oxidation/corrosion. 



31 




Figure 13. Cross section of Britannia Plate 
(backscattered electron image). 



An interesting analytical problem was posed in the examination and analysis of a 17th 
century pewter plate found at the bottom of the Caleb Pusey house well. Winterthur Museum 
analyses (J. H. Carlson) showed that the badly corroded plate contained copper ranging from 
2 to 20 percent (depending on the area analyzed). However, on a relatively clean, non- 
corroded area, a good approximation of the content of the base alloy was obtained (Sn = 95.1 
percent, Cu = 3.43 percent, Pb = 0.43 percent, and Sb = 0.62 percent) without resorting to 
cleaning or scraping the plate. The variability of the copper elsewhere on the plate was 
puzzling but it was conjectured that an electrochemical displacement mechanism had deposited 
copper on the plate as tin corroded. Carlson exposed pewter billets to an aqueous solution 
of CuSOit, Cu(0Ac)2 and acetic acid and confirmed that dissolved copper salts can be reduced 
from slightly acid water to deposit in the metallic state on pewter. Subsequent examination 
by the writer of a coppery-colored flake spalled off the Caleb Pusey plate revealed copper, 
iron and sulfur in the deposit (fig. 14). It was concluded that a chalcopyritic layer had 
formed on cathodic regions associated with areas in the plate that had undergone localized 
corrosion during the long exposure of the plate to the well water. 




10 


750EV 


0INT 


11 


1200EV 


9INT 


12 


1250EV 


13INT 


13 


1400EV 


172INT 


1^ 


1700EV 


41INT 


15 


2700EV 


0INT 


16 


2250EV 


12267INT 


17 


2700EV 


0INT 


18 


3000EV 


0INT 


19 


3250EV 


lINT 


20 


3700EV 


28INT 


21 


0EV 


0INT 


22 


4450EV 


0INT 


23 


5L00EV 


0INT 


2^ 


5200EV 


0INT 


25 


5700EV 


0INT 


25 


6400EV 


8047INT 


27 


6750EV 


4INT 


28 


7450EV 


15INT 


29 


8000EV 


6047INT 



Figure 14. Corrosion product on old pewter plate: (a) isolated spall, (b) EDXA analysis 
of spall; columns left to right are atomic number, x-ray energy in electron volts and 
total integrated x-ray count. 



32 



Wrought Iron Nails 

Nails are common finds for archaeologists not only at colonial American sites but 
also at all Roman occupation sites. Perhaps because of their commonness there have been 
very few attempts, either in Europe or here in the United States, to study ancient nails 
in any way other than by classification according to size and form. However, in 1962, 
Angus, Brown, and Cleere £21] had the opportunity to carry out metal 1 ographic examinations 
of nails found in a Roman legionary fortress (built about A.D. 85) at Inchtuthil, Perth- 
shire (England). Over 875,000 nails varying from 2-1/2 inches to 15 inches in length were 
found in a 12-foot deep pit. At the time of discovery, the nails received considerable 
publicity because of the excellent state of preservation of many of them. It was soon 
shown, however, that the Roman iron makers did not have any secret of producing rusting- 
corrosion-resistant iron. Actually, the main core of nails was protected over the centuries 
by the virtually anerobic conditions developed within the mass as the nails on the outside 
of the mass corroded rapidly and formed a protective, almost impermeable crust. The 
authors point out also that corrosion of the internal nails was further inhibited by the 
thermal oxide scale present on most of them. Where these coatings were discontinuous, 
preferential and severe corrosion attack had taken place. 

Similar observations have been drawn by the writer in examination of nails from 
several 17th century house sites in the United States. For example, the rose head nail 
shown in figure 15 was excavated by L. T. Alexander at the "Buck site" in Chestertown, 
Maryland. It is a typical flat-pointed nail of this early period, but in remarkably good 
condition. In cross section, it can be seen (fig. 16) that the nail is sheathed with a 
heavy, uniform layer of oxide (a-Fe203 by analysis) and it is believed that the barrier 
properties of this adherent oxide scale account for the preservation of this particular 
nail. It is conjectured that the protective oxide scale was formed in a house fire, 
possibly an intentional one, as it is known that wooden buildings in this period were 
occasionally set on fire for easier recovery of the nails whose value was considerable at 
that time. (But still much less than Roman nails in A.D. 85 which were prized by the Scot 
tribes more highly than silver or gold, because the iron could easily be converted into 
weapons [21].) 




Figure 15. Wrought iron nail from 
17th century archaeological site 
in Chestertown, Maryland. 




Figure 16. Wrought iron nail 
(Maryland) showing protective 
iron oxide scale. 



33 



It is of interest that this Maryland nail was analyzed by optical emission spectro- 
graphy and found to be different from a similar rose head nail found at the Caleb Pusey 
house (Pennsylvania) excavations in these two respects. 



Table 2. Emission spectrographic data for Maryland and 
Pennsylvania nails given in parts per million 
or percent. 

li Zn 

Md. Nail 0.5-2% 2 - 10% 

Pa. Nail 200 - 1000 0.2-1% 



The higher levels of titanium, but particularly zinc in the Maryland nail suggest 
that the iron (ore) for this nail came from Cumberland County, Pennsylvania, where the 
ores are zinc-containing. 




Figure 17. Wrought iron nail with defective oxide scale (Caleb 
Pusey House, Upland, Pennsylvania). 



Figure 17 is a longitudinal section of the Pennsylvania nail mentioned. As can be 
seen, the oxide scale is imperfect and pockets of rust were present at these surface sites 
where the protective scale was defective. The metallography of this nail was that of 
typical wrought iron of the period with characteristic mottled slag inclusions present in 
a pure iron or ferrite matrix. Figure 18 is included to illustrate again how, with the 
scanning electron microscope and using the technique of element distribution mapping, it 
is possible to delineate the partition of Fe, Si and P in the slag inclusions. Compar- 
ative studies (metallography and spectral analyses) of early wrought iron nails and of cut 
nails in their transition period of manufacture and acceptance can provide useful infor- 
mation on the history of local iron making practice. The nail from the Caleb Pusey house 
was "headed" cold or only warm at the best. The microstructure of the nail shown severe 
plastic deformation residual from forging, indicating that the forging temperature was not 
high enough to recrystal 1 ine the ferritic microstructure. 



34 




(c) (d) 



Figure 18. Wrought iron nail (Pennsylvania): 

(a) secondary electron image, 

(b) FeKa x-ray image, 

(c) SiKa x-ray image, 

(d) PKa x-ray image. 



Summary 

There is no doubt that most of the tools and techniques in use and available to the 
corrosion scientist can find applications in the study and conservation of metallurgical 
artifacts. When the archaeologist or metals conservator is uncertain about a course of 
action to follow, or the potential usefulness of a specific technique in characterization, 
he is advised to consult an individual or laboratory whose professional work involves the 
technique(s) in question. In effect, he is advised to continue the dialogue fostered at 
this meeting. 



35 



The writer must necessarily apologize for presenting only an abbreviated overlook of 
the subject of corrosion product characterization which, covered comprehensively, would 
require an entire book. The intention here has been to illustrate, in some personal study 
cases, that structural and compositional characterization of metals and their corrosion 
products can not only provide information on the early technology of manufacturing metal 
objects, but additionally, reveal the record of their subsequent history in terms of the 
changes wrought by long-term environment corrosion and oxidation. A physical and chemical 
record of this history can be read through the use of appropriate characterization and 
analytical techniques. 

The writer is in complete agreement with this expression of Dr. James A. Charles in 
concluding his Sir Robert Home Memorial Lecture on "Arsenic and Old Bronze" [22]: 

It is my view that practical field archaeology and subsequent artifact examina- 
tion today must combine a whole range of skills if the best possible results are 
to be obtained. It is my happy experience that active and informed participation 
on the part of interested scientists is welcomed and that it can be a stimulating 
and intellectually rewarding part of one's professional life. 



References 

[I] Pinkerton, J., An Essay on Medals, 3rd Edition, published for T. Cadell and 
W. Davies, Strand, London, 1808. 

[2] Gettens, R. J., Patina: Noble and Vile, in Art and Technology, (M.I.T. Press, 1970). 

[3] Cotton, J. B., Corrosion Technology, 141-145, May 1956. 

[4] Corrosion, 12^, 6, (June 1973). 

[5] Robinson, A. L., Science, 191, 1253-1256 (March 26, 1976). 

[6] Zavitsanos, P. D. , Carbon, 6, 731 (1968). 

[7] Brech, F. and Young, W. J., Application of Science in Examination of Works of Art 
(Boston, Museum of Fine Arts, 1967), p. 230. 

[8] Beck, C. W., Ed., Archaeological Chemistry, in Advances in Chemistry Series 138, 
American Chemical Society, Washington, D.C., 1974. 

[9] Kruger, J. and Frankenthal , R. P., in Physicochemical Measurements in Metals Research, 
Techniques of Metals Research, Vol. IV, Part 2, {interscience , 1970), 571-667. 

[10] Fleming, S. J., Authenticity in Art, in The Institute of Physics, London, 1975. 

[II] Vaughn, D. A., Corrosion, pp. 55t-58t (Feb. 1963). 

[12] Barker, H., Application of Science in Examination of Works of Art, (Boston, Museum of 
Fine Arts, 1967), pp. 218-221 . 

[13] Brothwell, D. , Science in Archaeology, D. Brothwell and E. Higgs, Eds. (Praeger Pub- 
lishers, 1970) pp. 564-569. 

[14] Hanson, V. F., Applied Spectroscopy , 27_, 5, 309-334 (Sept. /Oct. 1973). 

[15] Heinrich, K. F. J., Scanning Electron Probe Microanalysis, in Advances in Optical and 
Electron Microscopy, Barer, Rand Cosslett, V. E., eds.. Vol. VI, (Academic Press, New 
York; 1975) pp. 275-301. 

[16] Corrosion, 15^, 4, 199t-201t (April 1959). 

[17] Schmidt, M., J. Inst. Metals, 98, 238 (1970). 



36 



[18] Werner, 0., Praktisohe Metallographie , 4, 1, 3-15 (January 1967). 

[19] Verink, E. D. , Jr. and Heidersbach, R. H., Jr., Localized Corrosion - Cause of Metal 
Failure, ASTM STP 516, American Society for Testing and Materials, 1972, pp. 303-322. 

[20] Albrecht, J. F., Bulletin of the Arahaeologioal Sooiety of Delaware, No. 9, New Series, 
Spring 1972. 

[21] Angus, N. S. , Brown, G. T. , and Cleere, H. F., J. iron and Steel Inst., 956-968 (Nov. 
1962). 

[22] Charles, J. A., Chemistry and Industry, 470-474 (June 15, 1974). 



Discussion 

v. T. Chase: The identification of all the phases in a corroded metal artifact, such as 
the object from Hasanlu which you showed, is particularly useful. Have you published this 
any place? If not, I suggest that you include a couple of pictures, with the corrosion 
phases identified in the proceedings of this meeting. 

ii/. A. Nielsen: At the present time there are no plans to publish this material but, 
perhaps I could follow up on your suggestion. 

M. Goodway: What is the beam size of the ion scattering technique? 
D. Newbury: About 100 micrometers. 

C. S. Smith: The fine photomicrographs shown by Dr. Nielsen make it obvious that micro- 
structure must be considered when interpreting any gross measurements on corroding or 
corroded materials. Composition, especially when determined on the small samples that are 
invited by today's ultra-sensitive techniques, is a function of location. The results of 
both electrolytic or ion stripping techniques, commonly interpreted in terms of depth 
often depend more on surface distribution than on true variation with depth or thickness. 

N. A. Nielsen: I do not believe I have any further comments. I agree with what Professor 
Smith said. 



37 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 

PRINCIPLES OF GASEOUS REDUCTION OF CORROSION PRODUCTS 



C. Ernest Birchenall 

University of Delaware 
Newark, Delaware 19711 

and 

Russell A. Meussner 

Naval Research Laboratory 
Washington, 0. C. 20375 



1. Introduction 

All metals except gold eventually form oxides and, in some cases, sulfides or other 
compounds because those compounds are more stable than the metallic elements when exposed to 
the atmosphere. For many metals, the reaction takes place very slowly at or near room 
temperature, but the rates and the details of the process often are very sensitive to small 
concentrations of reactant materials in the environment. Old objects that have been in a 
slowly changing environment or that have spent long periods in a succession of different 
environments may bear clues to the history of the object in the nature of the reaction 
products. Removal or reduction of the reaction products may obliterate that evidence. 
Although the appearance of the object may be closer to its original appearance, it is again 
in a reactive state. 

This paper does not consider whether an object should be treated by gaseous reduction, 
but only summarizes the principles of equilibrium that place limits on the conditions for 
reduction. In addition, explicit discussion is included concerning reduction of the products 
of iron oxidation. This latter subject is interesting for many reasons; it has been studied 
intensely as a possible method for obtaining iron directly from its ores. In spite of many 
such studies, there is no simple, sure way to obtain dense iron by direct reduction of its 
oxides. 



2. Stability of Oxides 

Chemical equilibrium at constant temperature and total pressure is characterized by a 
minimum value of the Gibbs free energy G of the system. If the reaction is written as a 
balanced equation, the change in Gibbs free energy AG is the weighted sum of the molar Gibbs 
free energies of the product molecules minus the same sum for the reactants, where the 
weighting of the sums is in proportion to the coefficients in the balanced chemical equation. 
The minimum for the Gibbs free energy means that aG is zero for a very small displacement of 
the reaction forward or backward when all reactants and products are present in their 
equilibrium concentrations. 

AG is defined in terms of the more familiar quantities enthalpy (or heat content) H and 
entropy S by the equation 



AG = AH - TaS (1) 



where the difference A always is taken between the weighted sum of the molar property for 
products less the weighted sum of the same property for reactants. The enthalpies of com- 
pounds are expressed conveniently as their heats of formation from the elements so that 



39 



reactants and products both have values based on the same reference states. Thus, for the 
oxide alumina 



4/3 Al + O2 = 2/3 AI2O3 



(2) 



the enthalpy change in this reaction carried out at constant T and P is the heat of formation 
per mole of oxygen. It is advantageous for the comparison of relative stabilities of the 
oxides to write all equations for the same amount of oxygen. The standard value of any 
property change is obtained when the metal and oxide are pure and the oxygen is at one 
atmospheric pressure. These standard reference states are designated as having unit activity, 
and thermodynamic quantities measured with all reactants and products in their standard 
states are identified by a superscript zero. For the general case of oxide formation, the 
standard Gibbs free energy of formation is very simply related to the equilibrium partial 
pressure of oxygen that is simultaneously in equilibrium with the metal and its oxide, that 
is to the dissociation pressure of the oxide. The greater the dissociation pressure, the 
lower is the stability of the oxide and the less negative is its standard free energy of 
formation. 

Writing a generalized equation for the formation of an oxide: 



xM + O2 



Mx02 



(3) 



for which the equilibrium constant is 



(4) 



because the oxide and metal are in their standard states which have unit activity, x can be 
either a simple fraction or small integer. The equilibrium constant is related to the 
standard Gibbs free energy change for the reaction by 



AG° = -RT In K 



RT In PQ^ieq) 



(5) 



This equation is the basis for the plots of aG° versus temperature given in figure 1 for 
oxides and in figure 2 for sulfides. 

Unless the equation is given for formation of a higher oxide by oxidation of a lower 
oxide, the oxide is formed from the metal and oxygen, and the metal and oxide can exist in 
equilibrium at the dissociation pressure. These diagrams are known as Ellingham diagrams. 
Analogous plots have been assembled for halides, nitrides, carbides, eta., and they have 
interesting and useful properties, especially when supplemented by nomographic scales as is 
done in figures 1 and 2. 

The standard heat of formation aH° varies only slowly with temperature, and nearly 
linearly. The standard entropy change aS° also changes very slowly with temperature; this 
contribution is dominated by the change in the number of moles of gas between reactants and 
products because the molar entropies of gases are so much larger than those of the condensed 
phases. aS° per mole of oxygen immobilized in the formation of a solid oxide is about 190 
joules per mole per kelvin unless the metal or oxide is volatile. As a consequence, the 
lines for solid metals forming solid oxides or for lower oxides forming higher oxides are 
roughly parallel to one another because the slope is determined by TaS°, which is about the 
same for all of these cases. The more stable oxides with large negative heats of formation 
and low dissociation pressures lie below the less stable oxides on the Ellingham diagram. 



40 




Temperature, (°C) 



Figure 1. Ellingham diagram for oxides. Based upon the diagram first 
prepared by Richardson and Jeffes [1]^ and modified by Darken and Gurry 
[2]. A few additions and modifications have been made to emphasize 
the metals used most in old artifacts. 



The lines show small changes in slope where a metal or oxide melts and still larger changes 
in slope where a metal or oxide vaporizes. Analogous statements can be made for the sulfides. 



Figures in brackets indicate literature references at the end of this paper. 



41 




-10 



-20 



0 500 
Ph2S/Ph2 



Pe , atm 



10 



rlOO 



10 



■50 



10 



■30 



Temperature, (°C) 



Figure 2, Ellingham diagram for sulfides from the same sources as 
figure 1, with modifications. 

The free energy change, or AG, for an arbitrary oxygen pressure P^, is given by 

up 



AG = RT In Po/Po,(eq) 
which satisfies the requirement that AG = 0 where P 



(6) 



P. (eq) 



It is practical to reduce 



O2 <J2 

Pq^ only to about 10"^ atm by pumping and somewhat lower by the use of an active metal 

scavenger {e.g., Al , Mg, or Ti). However, to maintain good control of low oxygen partial 
pressures, mixtures of hydrogen and water vapor or carbon monoxide and carbon dioxide fre- 
quently are used. The lines for these equilibria have similar slopes because three moles of 
gas react to form two moles. However, oxidation of graphite by one mole of oxygen to form 



42 



two moles of carbon monoxide has a distinctly different slope than the metal oxidation lines 
in figure 1. In fact, this reaction has a negative slope, which is most important for 
reducing oxides. These three reactions are written below, and the equilibrium constants are 
written to show how the oxygen partial pressure can be controlled by maintaining the ratios 
of water vapor to hydrogen, carbon dioxide to carbon monoxide or by controlling the carbon 
monoxide pressure over hot graphite. 

2H2 +02= 2H2O (7) 



_ 1 



O2 ['H,0''Hr, 



2C0 +02= 2CO2 (10) 



'^2C " ^^C02/''C0 V (11) 



^02 



2C + O2 = 2C0 (13) 



Figures 1 and 2 have been fitted with nomographs. The index marks on the left-most 
scale of figure 1, labeled 0, H, and C, are to be used with the scales at the bottom and on 
the right for Pn , Pm n/Pu and Pm /Pms respectively. For example, if you wish to 

U2 n2U n2 HJ2 

determine the dissociation pressure of NiO at 1000 °C, place a straight edge at the 0 index 
mark, run it through the intersection of the NiO line with the 1000 °C vertical line, and 

extend the straight edge until it intersects the right-most line at P^. equal to just less 

U2 

than 10"i° atmospheres. Similarly, the mixture of carbon dioxide and carbon monoxide in 
equilibrium with Si02 and silicon at 900 °C can be found by running a straight line from the 
C index through the 900 °C point on the Si02 line and reading a ratio of P^q to P^q from 

the nomograph line closest to the right boundary of the graph equal to less than 10"^. 
Such a ratio would be very difficult to establish and maintain. 

In practice, hydrogen gas can readily reduce those metals that lie above the line 
marked H2O or slightly below it if the water is continuously removed. Similarly, CO can 
reduce about the same metals, although metal with dissolved carbon, or even carbides in some 
cases, may be the solid product of this reduction. The only reducing agent that becomes 
increasingly powerful with increasing temperature is carbon, usually introduced in the form 
of graphite or charcoal. Note that in the neighborhood of 2000 °C and above, such a stable 
oxide as AI2O3 should be reduced by carbon to form CO. In a sense, the reaction is even 
more favorable than the figure indicates because aluminum is volatile below 2000 °C. 
However, aluminum also forms gaseous suboxides that make its separation as metal very unlikely. 

The point that historians have drawn from the principles embodied in figure 1 is that 
the metals became available for use as soon as furnaces could be built to provide the tempera- 



43 



ture at which carbon would reduce the oxide, provided only that the ore was sufficiently 
abundant to be tested. The sulfide ores were no problem. Although figure 2 shows that CS 
is not stable enough to be useful directly, the sulfides could easily be roasted to form 
metallic oxides and SO2 , after which carbon could be used to reduce the oxide. 

Figure 1 shows that gold has no stable oxide at or above room temperature. The oxides 
of silver and mercury are readily decomposed in air by a small temperature increase. The 
oxides of copper, lead, nickel, tin, and iron can be reduced at reasonable temperatures by 
either hydrogen or carbon monoxide. Build-up of water vapor can be prevented by condensing 
out the excess water or by reacting it with a more reactive metal. Build-up of carbon 
dioxide can be prevented in the same way, or by supplying solid carbon at a location where 
it can react with the hot gases. 



3. Reduction of Oxides on Iron, Steel, and Cast Iron 

Figure 1 shows that the reduction of the lower oxides of iron requires either a tempera- 
ture in excess of about 400 °C or hydrogen that is thoroughly dried, that is, hydrogen with 
a low "dew point." For example, the line connecting the H index to the FesOi^ line at about 
250 °C shows that the hydrogen can contain only about 0.1 percent water vapor and remain 
reducing. Furthermore, at such a low temperature the reaction would be very slow. Slow 
reaction may be an advantage in the early stages of treatment of a corroded object in order 
to avoid physical damage from fast evolution of water vapor. Consequently, the reduction 
process might be started by heating slowly in a stream of pure hydrogen. 

However, steel and cast iron are alloys of iron and carbon. Many of these alloys were 
made intentionally with a nonuniform distribution of carbon resulting from a series of 
carburizing and oxidizing heat treatments, by hot forging high carbon alloys together with 
wrought iron, and by a variety of other laborious operations which were parts of a conscious 
art. The corrosion processes differentially attack these alloys. In the case of cast 
irons, prolonged corrosion, especially in seawater, may leave behind a residue of graphite 
infiltrated by silt, mud, or other matter in such a way that the form of the original object 
is almost wholly or just partially preserved. Much or all of the iron may have been dissolved 
by the electrolyte. In any of these cases, it is important to determine the condition of 
the object and to remove any samples that are to give information about the initial alloy 
content or its thermal or mechanical treatments before any high temperature reduction is 
attempted. 

Hydrogen is a decarburizing agent for carbon dissolved or included in the steel or iron: 

C(pg) + 2H2 = CH^ . (16) 

Mixtures of carbon dioxide and carbon monoxide may be either carburizing or decarburizing. 
Strongly oxidizing mixtures always decarburize, but reducing mixtures may carburize or 
decarburize depending on the carbon activity already in the alloy. Thus, the iron-to- 
carbon ratio may be disturbed severely as a result of corrosion but, because of the low 
temperatures, only to the depth that corrosion has penetrated. After high temperature 
reduction, however, the carbon content may be modified to much greater depths owing to the 
high diffusivity of carbon and of hydrogen. Corrosion followed by reduction may convert 
cementite (FesC) in some steels and cast irons into graphite. 

Changes in composition as a result of corrosion and subsequent reduction of products 
are not limited to steels and cast irons. Both high temperature oxidation and aqueous 
electrochemical corrosion of all alloys selectively attack the more reactive components. 
They may form insoluble compounds in situ or dissolve away preferentially with more noble 
constituents later being attacked and removed. When reduction is attempted, the compounds 
of the more noble elements that still are a part of the object are redu-^ed preferentially. 
Indeed, compounds of the most active elements may not be reduced at all. The sequence of 
these processes applied to the surface of an alloy that had been melted and cast, possibly 
hot or cold forged in addition, results in a surface region that is metallurgically dif- 
ferent than the original if it is not chemically different as well. Any reduction heat 
treatment that exceeds the recrystall ization temperature would remove evidence of cold work 



44 



and of quenching and tempering irreversibly. For example, cored brasses and bronzes, espe- 
cially those in which deep-seated corrosion products replaced one phase preferentially, 
should acquire new surfaces that are chemically different than the original surface. However, 
the reduced object should look more like the original than the corroded object did, and the 
removal of deleterious impurities like halides and sulfur-containing ions should contribute 
to stability against further degradation, provided that additional protective measures are 
taken. In any case, the chemically and metal lurgically altered surface region should be 
used only with great caution in interpreting the life history of the object. 



4. Oxidation Products 

Both copper and iron have multiple states of oxidation. The higher valence states are 
more stable relatively at low temperatures, and the relative stability of the lower valence 
states increases with increasing temperature. Dry oxidation of copper yields a layer of 
cupric oxide in contact with air. Valensi [3] found that cupric oxide occupied most 
of the thickness at low temperature, diminishing with increasing temperature until the 
dissociation pressure of cupric oxide reaches the ambient oxygen pressure, beyond which only 
cuprous oxide forms. When the environment contains water or water vapor, the product of 
oxidation may be a hydroxide or hydrous oxide at low temperatures, but is almost invariably 
anhydrous oxide at high temperatures. 



Table 1. Properties of Fe, Cu, and some of their oxygen-containing compounds. 



Material 



Structure 



a-Fe 
Y-Fe 

FeO 
FegOi, 
ct-Fe203 
Y-FeaOg 

a-FeOOH 
3-FeOOH 
Y-FeOOH 
Fe(0H)2 



b.c.c. 
f .c.c. 

NaCl 
Spinel 
a-Al203 
"Spinel" 

a-FeaOs 

Y-Fe203 



Vol/mol Fe 



,cm^ 



AV-reduced 
(%) 



7.1 
6.9 



11 .9-12.5 
14.9 
(15.2) 



40-43 
52 
53 



21 .3 
27.5 
22.4 
25.4 



67 
74 
68 
73 



Vol/mol Cu 

Cu f.c.c. 7.1 

CU2O Cuprite 11.6 39 

CuO Monocl. 12.2 42 

Cu(0H)2 29.0 76 



Iron shows a similar behavior. Some information about its oxides, as well as those of 
copper, is collected in table 1. It is particularly important to note, from the last two 
columns, that the oxides are much more voluminous than the metal from which they form. 
Figure 3 shows some of the chemical interconversions among iron, its chlorides, oxychloride, 
oxides, hydrous oxides, and hydroxides. At high temperatures, water and halogens are 
readily driven off the compounds, to leave only anhydrous oxides, if there are no physical 
barriers that prevent departure of the volatile products. Below the eutectoid temperature 
in the Fe-0 system at 570 °C, ferrous oxide (wiistite) is neither a product of oxidation nor 
a step in reduction of the higher oxides. 



45 




Figure 3. Relations among some iron compounds, adapted from A. F. Wells, Structural 
Inorganic Chemistry, 3rd ed., Oxford University Press, p. 545 (1962). y forms are 
cubic, a forms are rhombohedral or hexagonal. 



5. Reduction of Iron Oxides 

Direct reduction of the common oxides of iron, especially magnetite and hematite, 
which are present in the high grade ores or easily produced in beneficiating the lower grade 
ores, is a subject of intense interest for possible commercial exploitation. The main 
drawback to this process is the spongy, reactive nature of the iron that results from re- 
duction. Those who have used the process for conservation of objects of historical value 
have recognized this problem and taken steps to prevent reoxidation by the use of vapor 
phase inhibitors and filling the pores with waxes and resins. 

Failure to convert the oxides back to solid metal may not be entirely negative however. 
Many of the objects have lost iron by dissolution or by conversion to gelatinous products 
that are washed away. Graphite residues of cast irons and some steels may retain the shape 
of the original object even when most of the iron has been leached away. If little iron 
remains, there is no benefit to be obtained from a reduction process, although heating in an 
inert atmosphere may remove undesirable impurities. Some form of stabilization of the 
porous graphite should be provided. But if iron compounds are the major residue, they also 
may retain the shape and even the approximate size of the original object. Reduction to a 
solid mass would change the shape and size greatly, as indicated by a comparison of molar 
volumes as given in table 1, but the porous mass of iron that results from reduction occupies 
roughly the same volume as the compounds from which it is reduced. The purpose here is to 
review very briefly what has been reported about the kinetics of reduction of iron oxides. 

Any heterogeneous reaction requires a nucleation stage, the formation of the first 
particles of the product. If there is an unreacted metal core, Gellner and Richardson [4] 
showed that reduction of iron oxides under conditions that are very close to equilibrium 
could avoid the formation of additional nuclei. This permits deposition of iron back onto 
the unreacted core. This process may depend on the ability of hydrogen to diffuse rapidly 
to the metal-oxide interface or on the creation of a generally oxygen-poor oxide which can 



46 



reject iron only where it is in contact with the pre-existing phase. This condition is 
difficult to maintain, and others [10] have not been able to reproduce it. That is, very 
careful control of the oxygen partial pressure in the gas phase is necessary to avoid the 
formation of other metal nuclei where the reducing gas is in contact with the oxide. Indeed, 
maintaining such control over the whole surface of a large, irregular object probably is 
not feasible. 

It is to be expected that reduced iron nuclei will form at the external surface of a 
corroded object immersed in hot, reducing gas. The thin nuclei may grow until they impinge 
in the surface layer. In general, there will be a sequence of reducing gas, metal, lower 
oxide, higher oxides from every exposed surface. Wherever oxide is in direct contact with 
the reducing gas, it will tend to be covered with fresh metal. On the other hand, where 
metal covers the oxide, hydrogen from the reducing gas dissolves in the metal, diffuses 
rapidly through it to the metal-oxide interface where there is a strong driving force for 
the formation of water vapor. It may be the evolution of this gas that continuously deforms 
and fissures the solid layer, leading to the production of a porous, spongy mass instead of 
dense, solid metal. Hydrogen diffuses in through the iron much faster than oxygen can 
dissolve from the oxide and diffuse out, but the ability of water vapor to nucleate at the 
interface has not been demonstrated. Sjostrand [5] reports that the reduced iron is more 
porous below 570 °C where it forms from the higher oxides without an intervening layer of 
wiistite. 

The first reasonably complete studies of reduction kinetics of iron oxides have been 
made by McKewan [6], by Quets et al. [7], and by Hedden and coworkers [8,9]. They agreed 
generally about the kinetics and the resulting structures, although the mechanistic details 
of the process were not clear. When polycrystal 1 ine hematite is reduced in hydrogen, the 
metallic_ iron nucleates on the outer surface. Above 570 °C, or a little higher, a layer of 
porous wiistite is next, and a layer of magnetite lies between that and the unreduced hematite. 
Below 570 °C the wiistite layer is missing. Reduction of polycrystal 1 ine magnetite goes 
directly to iron below 570 °C, with an intervening wiistite layer somewhat above 570 °C. At 
the lower temperatures for both oxides, there is an induction period which is shortened by 
increasing the hydrogen pressure or the temperature, but extended by increasing the water 
vapor pressure in the reducing atmosphere. The induction period is shorter for magnetite 
reduction than for hematite reduction. It appears to be a period during which reduction 
propagates rapidly down grain boundaries in the oxide and establishes a pattern of cracks 
and rifts in the grains. After this period, almost instantaneously at high temperatures, 
the rate of reduction becomes constant until reduction is more than two-thirds complete in 
magnetite and more than nine-tenths complete in hematite. The linear reaction rate constant 
has the form [8] 

^l(^H, - ,,,, 
where K is the equilibrium water vapor to hydrogen pressure ratio for the reaction 



1/4 Fe304+ H2 = 3/4 Fe + H2O (18) 



Above 300 °C, this ratio is given approximately by 



K = 36 exp(-8000/RT) (19) 



For magnetite reduction in one atmosphere pressure of hydrogen, rg varies from about 
10-5 gy<ams of oxygen per cm^ per second at 400 °C to about lO"** gg/cm^-s at 550 °C. Allowing 
for some irregularities between 600 and 700 °C, rg approaches 10"" go/cm^-s at 1000 °C. 
These values were measured on magnetites with columnar grains that had cross-sectional areas 
averaging 0.035 mm^. The rates are slower if the water vapor formed in the reaction is not 
swept away quickly or if the grain size is larger. 



47 



McKewan [6] noted that when q ^'^ ^° small that the last term in numerator and denomi- 
nator can be neglected, the remaining equation has the form of a Langmuir adsorption isotherm. 
k2 is nearly independent of temperature, and kj has a temperature dependence that yields an 
activation energy between 40 and 60 kilojoules per mole per kelvin (9.55 and 14.33 kilocalorie 
per mole per kelvin). On this basis, he concluded that the rate is controlled by weak 
chemisorption of hydrogen which provides low coverage of the reactive surface at low 
hydrogen pressures and complete saturation at high pressures. 

The introduction of water vapor into the reducing gas, or permitting it to accumulate 
as a reaction product, retards reduction. There are two possible contributions--an increase 
in the effective oxygen potential of the mixture, an equilibrium effect that decreases the 
reducing power, and also the hydrogen in the gas phase increases the diffusion back pressure 
for the escape (through pores and fissures in the solid overlayers) of the new water produced 
by reduction. The former (equilibrium) effect could be included explicitly in the calculation 
of the rate constant. The latter effect was demonstrated by showing that inert gases, 
nitrogen and argon, added to the gas stream decreased the rate of reduction; k3 decreases 
with increasing temperature, becoming negligible at high temperature, and yielding a formally 
negative value for 'activation energy'. 

Hedden and Lehman [8] found that this pattern of reaction kinetics, including the 
activation energy for kj, is about the same for a variety of n- and p-type semiconducting 
oxides with a wide range of heats of formation. They concluded that the basic rate con- 
trolling stages of the reduction mechanism are the same, independent of the nature of the 
oxide. Their study of magnetite reduction and, with Endom [9], of hematite reduction showed 
that there was a very strong grain size contribution to the rate. The finer-grained oxides 
were reduced much more rapidly than coarse-grained oxides because preferential reaction at 
grain boundaries and the formation of fissures there presented a surface area for reaction 
that was greater for the fine-grained specimens. Their synthetic oxides had columnar grains, 
which made the analysis of the problem simpler than it would have been if the boundaries 
formed a less orderly pattern. They were able to estimate the average thickness of the 
layer from which oxygen is depleted to be about 65 micrometers during the constant rate 
period independent of the temperature. This region appears to be microfissured to permit 
the removal of oxygen by the transport of water vapor. 




Figure 4. Porous iron resulting from the complete reduction of magnetite 
at 423 °C [9]. The average grain cross section is 0.297 mm [2]. 50X. 
(Reproduced from Endom, et. at. [9], p. 635.) 

Figure 4 shows a photomicrograph of a magnetite specimen that has been completely reduced 
to iron below 570 °C. The grain boundary fissures and porous grain interiors are easily 



48 



Figure 5. Hematite partially reduced at 590 °C [9]. The bright phase 
near the top is iron, the light gray interiors of the grains are 
hematite, and the dark gray bands along the grain boundaries are 
magnetite. The darkest area at the top is the mounting medium. 200X. 
Reproduced from Edom, et. at. [9], p. 642.) 

seen. Figure 5 shows a partially reduced hematite specimen with an external layer of iron, 
fissured at the hematite grain boundaries, and broad magnetite envelopes growing into each 
hematite grain after penetrating deeply along the grain boundaries. Wiistite, if present at 
the iron-magnetite interface, is not resolved. The temperature of reduction, 590 °C, is 
just above the eutectoid temperature where the wiistite composition range is small. The 
grain sizes of hematite used in these studies were about 15 times smaller than the magnetite 
grain sizes, which may account for larger values of ki for the magnetite reduction, although 
activation energies were about the same. The magnetite layer averaged about 10 micrometers 
in thickness at this temperature and was smaller as the temperature of reaction was lower, 
but in all cases the variation from place to place was large. In the photomicrograph, 
the magnetite layer is much thicker along the grain boundaries than on the surfaces where it 
is covered by a porous iron layer. Endom, Hedden and Lehman [9] note that for each eight 
iron atoms reduced to metal, the stoichiometric relations provide one iron ion that can 
diffuse through the Fesd^ layer to the magnetite-hematite interface where it can reduce 
4Fe203 to 3Fe304. This step probably controls the growth of the FeaO^ layer because magnetite 
is a sufficiently good semiconductor to transport the electron easily. Iron may be super- 
saturated in the magnetite at its interface with iron if the slowest step is there. 

At some temperature between 570 and 610 °C, wiistite intervenes as a product during 
reduction, and the reaction kinetics change markedly. Hydrogen reacts to form water initially 
at the wiistite-gas interface, then near the iron-wiistite interface. The rate increases with 
temperature to about 650 °C, passes through a minimum just above 700 °C, then rises again. 
Decreasing reduction rate with increasing extent of reduction at fixed temperature shows 
that diffusion resistance in the growing iron layer cannot be neglected. Grain boundary and 
pore diffusion affect the rates. Nevertheless, oxygen still appears to be transported from 
near the wiistite-iron interface as water vapor. Studies on the direct reduction of synthetic 
wiistite samples showed that the new features are derived from the appearance of wiistite 
among the intermediate products. 

The rate equation of Hedden and Lehman [8] probably is a satisfactory guide for planning 
a tentative reduction procedure for a corroded object, although the extended studies by 
Turkdogan and his coworkers [10] have shown that the real processes are considerably more 
complex. They found that porous small particles of hematite undergo reduction throughout 
the particles independent of size differences, especially at low temperatures. For larger 



49 



hematite particles, the early stages of reduction take place in the pore mouths near the 
iron-w'ustite interface. The boundary between the iron and wustite particles is not even; 
the time to reduce a particle completely is proportional to its diameter. However, control 
of the rate depends on both the rate of reduction at the interface and gaseous diffusion in 
the fine pores in the iron and wustite. For larger particles and higher temperatures, 
diffusion in the pores becomes rate-controlling, and the time to complete reduction is 
inversely proportional to the square of the particle diameter. As the reduction temperature 
decreases, the pore size in the iron decreases and its permeability to gases decreases. The 
pore volume is almost completely interconnected but not uniform in cross section. The 
surface area per unit mass of iron is much greater when it is formed by low temperature 
reduction. Not all of the pores seem to participate in the reduction process, but it may be 
that the gas approaches the equilibrium composition within those pores with little mixing. 
Even if the original hematite and magnetite are dense, the wustite that forms on them is 
porous, and iron grows inside the wustite as well as on it. It is proposed that reduction 
of the wustite takes place by oxygen diffusion through a very thin layer of iron so that the 
water molecules are desorbed from the iron surface. A mathematical analysis of a simplified 
geometrical model based on a single porous oxide appears to synthesize the elements of mixed 
rate control in a way that is numerically consistent with the experimental observations. 



6. Gaseous Reduction of Corroded Objects 

The complexity of both the corrosion process and the variable nature of the foreign 
matter that can accrue along with corrosion products, together with the incomplete under- 
standing of the reduction process, make it a hazardous task to formulate a recipe for treating 
or restoring specific objects. This summary is presented only as a check list to ensure 
that the conservator has thought through the possible pitfalls before embarking on a particu- 
lar course of action. 

1. External foreign matter must be removed by physical means or superficial 
chemical reactions. The usual silicates and alkaline earth compounds that consti- 
tute muds and silts are not reducible under moderate conditions. Any investigation 
or sampling that is desired for purposes of establishing microstructure and even 
some aspects of composition should be done before heating. Replacement of small 
divots may be undetectable after some of the more vigorous reduction treatments. 

2. The object should be heated gently to drive off physically and chemically 
included water without blowing off corrosion products and to minimize cracking from 
differential thermal expansion. Carbonates and sulfates may decompose, at least 
partially, and some volatile chlorides may be removed in this step. Removal of 
halogen and sulfur compounds could be the most important benefit in the whole 
process. In some cases, a mildly oxidizing atmosphere may be useful; in other 
cases, mildly reducing or inert gases may be better. 

3. If much of the metal core remains, initial reduction in a mildly reducing 
atmosphere, controlled H2/H2O or CO/CO2, "i^y encourage deposition on the existing 
metal in preference to forming new metal nuclei on the surface. When the atmo- 
sphere is made strongly reducing, metallic nuclei may form on any surface to which 
the reducing gas can penetrate, but particularly the outer geometric surface of the 
object. Metal grows from these nuclei; the original volume of the object is not 
restored precisely even when corrosion has dissolved away enough material that the 
expanded residue occupies the original volume. 

4. The freshly reduced product is highly reactive metal with a spongy struc- 
ture unless it is heated to a high enough temperature for extensive sintering. To 
prevent quick reversion to the oxidized state and to provide mechanical support for 
the weak, porous structure, some filling and sealing process must be used. This 
procedure usually should incorporate inhibitors. 

5. The most ambitious and successful practice of gaseous reduction methods 
appears to be the work of Barkman [11] and his associates in the Wasa preservation 
program. Their work includes many practical refinements that conservators would be 
wise to repeat unless a development program can be carried through to prove out variants. 



50 



6, Reduction of corrosion products seldom yields a consolidated metal object 
with the original dimensions. The porous, reactive metal requires protection to 
prevent immediate reoxidation. In some cases, most of the benefits of treatment 
might be obtained by careful oxidation to a state that is genuinely stable in the 
atmosphere against further degradation. Heating gently to drive off water, halides, 
and the oxides of carbon and sulfur, followed by oxidation in air or oxygen to a 
sufficiently high temperature to convert all accessible surfaces to a fairly thick 
layer of anhydrous oxide and to sinter that oxide into a continuous layer (perhaps 
under an inert or slightly oxidizing atmosphere) that would shield the unoxidized 
interior, should cause little additional increase in volume or loss in surface 
detail. Cooling must be done slowly to avoid cracking and spalling. Indeed, a few 
small cracks might have to be filled with some agent to prevent local corrosion. 
However, an object so treated should be nearly immune to further reaction because 
the outer layer would be in a thermodynamical ly stable state. The nature of the 
treatment should be evident in the product itself, and it would still be available 
for a future reduction treatment if that is considered desirable. 



References 

[I] Richardson, F. D. and Jeffes, J. H. E., J. Iron Steel Inst. 160, 261 (1948); 171, 167 
(1952). 

[2] Darken, L. S. and Gurry, R. W., The Fhysioal Chemistry of Metals , p. 349 and 361, 
(McGraw-Hill Book Co., New York, 1953). 

[3] Valensi, G., Vrooeedings of the International Conferenoe on Surface Reactions, p. 156 
(Pittsburgh, 1948). 

[4] Gellner, 0. H. and Richardson, F. D. , nature, 168, 23 (1951 ). 

[5] Sjostrand, E., in Bulletin No. 61 E, Korrosiansinstitutet , 8, (1973) 

[6] McKewan, W. M. , Trans. Met. Soa. AIME , 22]_, 140 (1961); 224, 387 (1962). 

[7] Quets, J. M. , Wadsworth, M. E., and Lewis, J. R. , Trans. Met. Soo. AIME, 218^, 545 
(1960); 221, 1186 (1961). 

[8] Hedden, K. and Lehman, G. , Arshiv. fur daa Eisenhuttenwesen, 34, 887 (1963). 

[9] Endom, A., Hedden, K. , and Lehman, G. , in Reactivity of Solids , G. M. Schwab, ed. , 
p. 632 (Elsevier Publ . Co., New York, 1965). 

[10] Turkdogan, E. T. and Vintners, J. V., Met. Trans. 2, 3175 (1971). 
Turkdogan, E. T. , Olsson, R. G. , and Vintners, J. V., ibid., 3189. 
Turkdogan, E. T. and Vintners, J. V., loa. ait. 3_, 1561 (1972). 
Tien, R. H. and Turkdogan, E. T., ibid., 2039. 

[II] Barkman, L., see paper in this symposium; also in the Bulletin cited in Ref. 5, 
page 14. 



Discussion 

P. Caspar: Can the change in slope of the rate vs. inverse temperature curve for reduction 
of magnetite and hematite at the temperature of stability of FeO be interpreted as being 
due to the involvement of FeO in the rate determining step? Is there a simple interpre- 
tation of the dramatic decrease in apparent activation energy for reduction above the 
stability point of FeO? 

R. A. Meussner: There have been many attempts to interpret that type of data, but I think 
that they are all questionable. The activation energies that they get for the low tempera- 
tures are in the order of 14000-15000 calories. In the low temperature sensitivity 



51 



region, the activation energy we are talking about is 3000. I do not believe one should 
try to interpret a process as complicated as this is, using data collected that way. 

C. S. Smith: When applying the high-temperature hydrogen reduction technique to objects 
of archaeological interest, it must be borne in mind that the resulting permanence is 
accompanied by the destruction of all internal records of the technical history of the 
iron. If they are not changed by subsequent heat and reduction, metallic microconstituents 
such as martensite or pearlite and particles of slag or other nonmetallic inclusions 
intimately reflect the conditions of refining, shaping, and heat treatment that produced 
the object. Even the external shape after reduction is only an approximation of the 
original, and the aesthetic quality of the surface, though it may be attractive, is quite 
different. Incidentally, a decrease of reaction rate with increasing temperature while 
the thermodynamic conditions remain unchanged is not uncommon when the temperature is high 
enough to cause sintering and hence a change in the geometry within which diffusion occurs. 
The hydrogen deoxidation of solid copper involving no phase changes shows a rate that 
decreases between 800 and 900° even greater than that observed by Dr. Meussner in his iron 
oxide system. 

E. V. Sayre: To your knowledge, have streams of atomic hydrogen been used, possibly at 
low temperatures, for the reduction of corrosion layers on metals? 

R. A. Meussner: I am sorry but I really don't know whether this has been tried or not. 

C. E. Birahenall: The Ellingham diagram, figure 1, when supplemented in the upper left 
corner by the curve for HgO, provides a very compact rationalization of the history 
of metallurgy. Those oxides that are most easily reduced, which have the least negative 
free energies of formation, were the first to yield their metals for man's use. 

Most of the reaction lines on Richardson and Jeffes' figure slope upward because the 
consumption of gaseous oxygen, combining with solid or liquid metal to form solid oxide, 
results in a decrease in entropy, hence less negative Gibbs free energy change with 
increasing temperature. The notable exception to this rule is the reaction of solid 
carbon with one mole of oxygen to form two moles of carbon monoxide. Only that curve 
slopes downward to more negative free energy change as temperature increases. As it 
crosses the metallic oxide lines it marks approximately the temperature range in which 
carbon becomes an effective reducing agent for that particular oxide. As man could build 
hotter furnaces, more stable oxides yielded their treasure. 

The Ellingham diagram for sulfides shows that carbon is not an effective reducing 
agent for sulfides. The stability of SO2 makes possible the roasting of sulfide ores to 
form oxides that can be reduced with carbon. Originally the two steps probably took place 
in the same fire. 

M. Pourbaix: It has been said during this presentation that hydrogen may be most helpful for 
reducing all of the oxides of iron to metal. Perhaps some other treatments, with ammonia, 
may lead to the formation of gaseous ammonium chloride. All such treatments may also be 
studied with the help of equilibrium diagrams. ^ 

Figure 6 shows as a function of the logarithims of the partial pressure in gaseous 
oxygen O2 and of 1/T the equilibrium conditions of the system carbon-oxygen. The slope 
of every line is the enthalpy of reaction, and the ordinate at origin, for the infinite 
temperature, is the entropy of reaction. Lines (b) represent the homogeneous equilibria 
between gaseous CO and CO2 for different CO/CO^ ratios; lines {a) represent the hetero- 
geneous equilibria between solid carbon and gaseous CO for different partial pressures 
in CO; lines (e) represent the heterogeneous equilibria between solid carbon and gaseous 
CO2 for different partial pressures in CO2. Lines (a) and {b) show the vaporization of 
solid carbon with formation of carbon vapor oi and 02- 



2See Cebelcor's Rapports Techniques 115, RT. 181183. See also A method of studying involved 
equilibria and its appliaations to metallurgical processes. Proceedings 11th International 
Congress Pure and Applied Chemistry, London, 17-6 July 1947, 5 871-889 (1953). 



52 




1 1 1 1 1 1 — I — I — I I I I 1 1 — n 1 1 

100 150 zoo 250 300 4 00 500 600 700 900 1,500 3,000 10,000 a> 



Temperature (°C) 

Figure 6. Equilibrium of the system C-0. 

Figure 7 shows a similar diagram for the zinc-oxygen system, with indication of the 
stability conditions of solid Zn, liquid Zn, gaseous Zn and solid ZnO. One may notably 
see on this figure that, when heated, solid ZnO decomposes along line (/) in gaseous zinc 
and gaseous oxygen according to reaction 2 ZnO ^ 2 Zn + O2, with a total pressure of 1 atm 
at 1949 °C. 

By superposing figures 6 and 7 one gets, with addition of equilibrium data for solid 
ZnCOg, figure 8 which relates to the ternary system Zn-C-O with consideration of solid C, 
Zn and ZnO, liquid Zn, and gaseous CO, CO2 and Zn. This figure shows notably that, by 
heating a mixture of solid ZnO and C, as is usual in the thermal metallurgy of zinc, one 
might get directly liquid zinc (corresponding to a zinc blast furnace) provided one could 
operate at 1052 °C and under a pressure of 7,8 atmospheres. 

It is possible to predict easily, with such diagrams, the conditions where any. high 
temperature reaction may occur, including those mentioned by Dr. Meussner for the removal 
of oxygen and of chlorine by hydrogen. 

Figure 9 shows an overall diagram for the system H-O. Figure 10 shows part of this 
diagram, figure 11 shows part of the Fe-0 diagram. Figure 12, obtained by superposing 
figures 10 and 11, relates to the H-Fe-0 system, and shows the conditions where FesOit, 
FeO and Fe may be obtained by reduction of Fe203 with H2/H2O mixtures. Figure 13 relates 
to the Fe-Cl system, and shows notably conditions where FeCl2 may be removed in the gaseous 
form. 

Many similar diagrams have already been set up about 40 years ago. We are now dis- 
cussing with some American friends (John Elliott at M.I.T., G. Simkovitch at State College, 
Earl Gulbransen at Carnegie Mellon) the possibility of preparing a series of "Atlas of 
Chemical and Electrochemical Equilbria in the presence of a gaseous phase" which would 
consist in a series of textbooks in inorganic chemistry for oxides, hydrides, chlorides, 
sulfides, hydrates, carbonates, sulfates, etc. John Chipman at M.I.T. has already 



53 




100 ISO 200 250 300 400 SOO 600 TOO 900 1,000 1,500 3,000 10,000 a> 

Temperature (°C) 
Figure 7. Equilibrium of the system Zn-0. 




150 200 250 300 100 500 600 700 900 1,500 3,000 I0,000 <» 

Temperature (°C) 



Figure 8. Equilibrium of the system Zn-O-C. 

54 




100 ISO zoo 250 300 400 SOO 600 700 9O0 1,500 3,000 10,000 

Temperature CO 
Figure 9. Equilibrium of the system H-0. 



400 500 600 700 800 900 1000 1200 1400 Temperature, "C 




1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 1/T x ifl-^ 

Figure 10. Equilibrium of the system H-0 (detail). 



55 



400 500 600 700 800 900 1000 1200 1400 Temperature, 




1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 1/T x 10 

Figure 11. Equilibrium of the system Fe-0 (detail). 

400 500 600 700 800 900 1000 1200 1400 Temperature, 




1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 Ml ^ 

Figure 12. Equilibrium of the systems H-0 and Fe-0. 

56 




2 L.'J 1 0,5 O 

■ I I I I I !_J I I 1 I I 1 I I I I I I 1 1 I I 




T 1 1 ' 1 ' 1 ' 1 ' — I — ' — I ' I ■ 1 — ' — I — >— TT 



ISO ZOO 30O 400 500 600 BOO 1,000 1,500 

Temperature (°C) 
Figure 13. Equilibrium of the system Fe-Cl. 

used such diagrams in 1964 for studying problems of thermal metallurgy related to tantalum, 
niobium and vanadium. I very much hope that the preparation of such Atlasses, which would 
be extremely useful for both teaching and research, will be starting in a near future. 
This is a work certainly for ten or twenty or thirty years. 



57 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gai thersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



SOME BRIEF REMARKS ON ELECTROCHEMICAL REDUCTION 

Jerome Kruger 

Institute for Materials Research 
National Bureau of Standards 
Washington, DC 20234 

The corrosion scientist uses electrochemical reduction for three reasons: 1) to 
remove oxide or other corrosion product films in order to do experiments that start with 
a bare metal surface, 2) to measure the amount of corrosion product present on a metal 
surface, and 3) to determine the nature of the corrosion products on a metal surface. 
Because all three of the above reasons for carrying out electrochemical dissolution are of 
value to conservators and archaeologists but with perhaps different objectives, these 
brief remarks are directed towards acquainting them with some of the approaches, concepts, 
and experience developed by corrosion scientists. By no means is this discussion intended 
to be comprehensive. Those needing further details can obtain them from the literature 
that will be cited at the end of this discussion. 

1. Chemical Reactions Involved 

The reduction of corrosion product film is a cathodic process, i.e. , electrons must 
be supplied. The following two equations represent such reduction reactions for one kind 
of iron oxide layer and for one kind of copper oxide layer: 

Fe203 + 3H2O + 2e' -> 2Fe'^'^ + 60H' (1) 

CU2O + H2O + 2e' 2Cu + 20H" (2) 

The electrons taking part in the above reactions are either supplied by a battery, 
the mode of carrying out an electrochemical reduction on which these remarks will con- 
centrate, or they can be provided by an anodic corrosion process represented by 

Fe Fe"^^ + 2e' (3) 

or 

Cu ^ + e". (4) 

The latter mode of reduction is called autoreduction and will be discussed briefly later on. 

There are two other cathodic reactions that should be mentioned because they can also 
occur along with the desired film reduction reactions (1) and (2) and interfere with them 
in some instances. These are the oxygen and hydrogen ion reduction reactions. 

O2 + 2H2O + 4e' ^ 40H" (5) 

2H^ + 2e" ^ H2 (6) 



59 



These are problem reactions which can occur concurrently with reactions such as (1) and 
(2). They therefore compete with the film or layer reducing reactions for electrons and 
thereby lower the efficiency of the film removal reduction process or prevent it from 
occurring at all. To lower the possibility of reaction (5) creating problems, a suitable 
experimental arrangement for carrying out an electrochemical reduction is necessary. This 
will be the topic of Section 2. Thermodynamic and kinetic considerations determine 
whether reaction (6) can be a problem. These will be discussed in Sections 3 and 4. 



2. Experimental Considerations 

The experimental arrangement for carrying out an electrochemical reduction is shown 
in figure 1. The arrangement shown is for carrying out a constant current reduction (gal- 
vanostatic) . 1 A detailed description of the experimental aspects of cathodic reduction 
can be found in a paper by Oswin and Cohen [2]. For the purpose of this discussion it 
will be sufficient to describe the different parts of the arrangement shown schematically 
in figure 1. The reduction cell will have to be tailored for the object to be cleaned by 




variable 
resistor 



Reduction Cell 



Figure 1. Schematic diagram of constant current reduction apparatus. 

reduction but it should also embody the features listed by Cohen and Oswin: a) means for 
excluding air; b) provisions for carrying out a complete flushing of the cell with an 
inert gas (usually purified nitrogen) with the specimen in place and immersed in a well 
de-aerated el ectrolyte^ ; c) an arrangement that keeps the anode as far as possible from 
the cathode (the specimen) to minimize oxygen diffusion to the cathode; and d) if a 
solution analysis for dissolved metal ions is desired, a small as possible cell volume. 
Items a) and b) listed above are incorporated to remove oxygen from the solution used to 
carry out the reduction in order to avoid the complications mentioned in Section 1 arising 
from reaction (5) . 

The electrical circuit, shown in figure 1, that is attached to the reduction cell is 
quite simple. The variable resistor controls the current to be applied; it determines the 



^It is possible to also employ a constant potential (potentiostatic) technique to reduce 
corrosion products. In this technique, the potential where reduction of a given corrosion 
product is known to take place is chosen. A potentiostat is needed when this technique 
is used. A description of the circuit for a simple potentiostat has been given by Green 
et al. [1]^. 

^Figures in brackets indicate literature references at the end of this paper. 
^See Oswin and Cohen [2] for a discussion of appropriate electrolytes to use. 



60 



rate of reduction. This current is supplied by the battery and measured by the micro- 
ammeter. The saturated calomel reference electrode measures the potential of the specimen 
where corrosion product films are to be reduced and allows one to determine when the 
reduction process is completed. By recording the potential of the specimen versus time 
(chronopotentiometry) one obtains a curve of the sort shown in figure 2. From this curve 
a number of valuable pieces of information can be obtained. The width of the upper 
plateau A is a measure of how much material is being reduced at the potential value where 



t 

+ 



Potential 
of 

Specimen 

vs. 
Reference 
Electrode 



reduction of corrosion 
product A 



reduction of corrosion 
product B 

C 



hydrogen ion 
reduction 



Time 

Figure 2. Hypothetical reduction curve for electrochemical reduction of corrosion 
product layer containing corrosion products A and B. 

the plateau is found assuming that the process occurs at 100 percent current efficiency. 
If the current efficiency is 100 percent, all of the electrons are going into reducing 
corrosion product A. Because the reduction is being carried out using a constant current 
for the experimental arrangement shown in figure 1, the product of the number of seconds 
making up the width of the plateau and the current chosen for the reduction gives the 
number of coulombs involved in reducing all of A on the specimen's surface. The amount of 
A can then be calculated from the formula 



^A^ ^A 



(7) 



where = mass of corrosion product A reduced, grams 

i^ = current (in amperes) involved in the reduction of A per unit area 
t = width of plateau in seconds 

E. = molecular weight of A (in grams) divided by number of electrons involved in 
the reduction of the metal atoms in A 

F = Faraday constant = 96500 coulombs. 



61 



The potential at which the plateau for A occurs can sometimes be used to determine 
the nature of A from a knowledge of the potentials of formation of various corrosion 
product species. When the reduction of A is complete, the potential changes and its value 
becomes more negative until, in our example, another plateau is reached where the reduction 
of corrosion product B commences. The potential and width of the B plateau reveal as 
before, the identity and amount of B. Usually the final plateau occurs where the hydrogen 
ion is the species reduced (plateau C). The potentials of the plateaus are determined by 
thermodynamic considerations, the concern of the next section. 



3. Thermodynamic Considerations 



A valuable aid for looking at the thermodynamic considerations governing electro- 
chemical reduction are the Pourbaix potential -pH diagrams [3]. These diagrams are de- 
scribed and discussed in the paper by Pourbaix occurring elsewhere in this volume. Such 
diagrams can be used to ascertain the potential of formation (or reduction) of the various 
oxides or sulfides making up corrosion product layers. Caution should be exercised, 
however, because kinetic considerations sometimes control the position of the plateau when 
nonequi 1 ibrium conditions prevail and cause the plateaus to occur at different values than 
those given in the Pourbaix diagram. In many instances, however, the diagrams can provide 
a useful guide to the corrosion products being reduced at a given potential. 

Equally important as the potentials of corrosion product reduction, are the parallel 
oxygen and hydrogen evolution lines, the upper and lower thin diagonal lines respectively, 
shown in figures 3 and 4. Above the upper line, oxygen evolution can take place, below 
the lower line, hydrogen evolution can occur. These lines (especially the hydrogen 
evolution lines) are of great utility in determining whether it is feasible to reduce 
corrosion products on a given metal. In figure 3 are shown the Pourbaix diagrams for six 
metals of interest to those concerned with studying or restoring metal artifacts. 





1 25.Iron| 




Figure 3. Potential-pH (Pourbaix) diagrams for easily reducible metals. Upper thin 
diagonal line is the oxygen evolution line, lower 1s hydrogen. The unshaded regions 
are those where the metal is the stable phase, the thinly shaded regions are those 
where corrosion takes place, and the coarsely shaded regions are those where corrosion 
products are the stable phases (from Pourbaix [3]). 



62 



I oZ.Zinc I 



0 14 

[35.Chromium| 



39. Aluminium 



14 



Figure 4. Potential-pH (Pourbaix) diagrams for diff icult-to-reduce metals. Upper 
thin diagonal line is the oxygen evolution line, lower is hydrogen. The unshaded 
regions are those where the metal is the stable phase. The thinly shaded regions 
are those where corrosion takes place, and the coarsely shaded regions are those 
where corrosion products are the stable phases (from Pourbaix [3]). 



The unshaded areas are those regions of potential and pH where the metal is stable. 
In these regions the metal is the thermodynamical ly stable phase; they are the regions 
where conditions exist to achieve electrochemical reduction. A crucial consideration is 
the location of the hydrogen line with respect to the regions where the metal is the 
stable phase. The diagrams reveal that, unsurprisingly, there is no problem for gold. 
Likewise, reduction is easy for silver. The corrosion products on copper are also easy to 
reduce. As one lowers the potential by applying a cathodic current, if the region where 
metal is stable can be reached before reaching the hydrogen line, then competition between 
the reduction of the hydrogen ion and the reduction of the oxide on the metal is won by 
the latter process and there is usually no problem in achieving an efficient reduction of 
corrosion products. For lead, especially in the middle pH regions, there is no problem. 
In the case of iron, the oxide reduction potential and the hydrogen evolution line almost 
coincide and, hence, there is no problem'*. The situation for tin is also quite good. The 
metals in figure 3 are metals where theoretically one has a good chance for removing 
corrosion products by cathodic reduction. For the metals shown in figure 4, however, the 
situation is different. For example, for zinc, the distance between the oxide reduction 
potential line and the hydrogen line is much greater than those in figure 3. For chromium, 
they are even further apart. The situation is even worse for aluminum. (It is because of 
these thermodynamic facts, among others, that these metals were not produced by the 
ancients.) Simply put, the electrons supplied by the battery will all be used to reduce 
hydrogen ions and will not be available to reduce the oxide that exists on a metal specimen. 
This would indicate that in the future when museum conservators try to restore aluminum 
beer cans, they will have to employ some other approach than electrochemical reduction. 



4. Kinetic Considerations 

The rate of corrosion product reduction is crucial in determining whether it is 
practical to use electrochemical reduction to remove corrosion products from a metal 
artifact. Thermodynamics determines what reactions are possible under a given set of 
conditions. Kinetics determines the rate of a reaction and whether, when two reactions 
are possible, the desired reaction (reduction of corrosion products) will predominate. 
For example, when both hydrogen reduction and corrosion product reduction are possible, it 
is necessary that most of the electrons supplied go towards reducing the corrosion product. 



k 

It should be noted that the Pourbaix diagram in figure 3 shows a direct conversion of 
corrosion product to metallic iron. As Oswin and Cohen [2] show, however, the corrosion 
product can be reduced first to ferrous ions (eq. (1)). This fact will become important 
in the determination of the current efficiency as discussed in the next section. 



63 



An important measure of the effectiveness of carrying out the corrosion product reduction 
process is the current efficiency. It can be determined from the following expression: 



Current efficiency = ^ ^-^ (8) 

m 

where = grams of metal produced by reduction of the corrosion product 
= equivalent weight in grams of metal 
F = Faraday's constant in coulombs per equivalent 
i = current used for reduction in amperes 
t = time of reduction in seconds. 

For a metal such as copper the corrosion product is reduced directly to the metallic 
phase (eq. (2)) and the metal is deposited on the specimen. For such a metal it is 
usually not possible to measure W^. For a metal such as iron the corrosion product is 
reduced first to a soluble species (eq. (1)) and the quantity of these can be measured by 
chemical analysis of the electrolytic solution. In this case Wp^ can be measured and the 
current efficiency determined from equation (8). For iron, where the current efficiency 
can be determined from the quantity of metal dissolved in the electrolytes some of the 
experimental conditions that can affect the current efficiencies can be listed as follows: 

1) Presence of oxygen dissolved in the el ectrolyte--if oxygen is dissolved in the 
electrolyte, the reduction of oxygen competes with the reduction of corrosion product. It 
is for this reason that the experimental arrangement shown in figure 1 includes provisions 
for working in an inert atmosphere. 

2) Electrolyte buff ering--Oswin and Cohen [1] have shown for iron that working in an 
unbuffered solution can give current efficiencies greater than 100 percent because in 
addition to adding metal ions by the reduction of the corrosion product process, additional 
ions are added because of corrosion. This occurs because of the process called "auto- 
reduction." This process results when, as mentioned earlier, reactions such as those 
given by equations (3) and (4), supply electrons that are in addition to the electrons 
supplied by an external source of current. A valuable discussion of autoreduction is 
given by Pryor and Evans [4]. Autoreduction can be decreased if a buffered electrolyte is 
used whose pH is in the neutral range of values. If, however, a buffered or unbuffered 
solution is used with a high pH, autoreduction is less of a problem, but other reactions, 
especially oxygen reduction, can become a problem. 

3) Presence of a complexing agent--a complexing agent such as a salt of ethylene- 
diaminetetraacetic acid is a chemical which complexes the metal ions that go into solution 
and ties them up so that they cannot use up current, for example, by electrodepositing on 
the surface of the specimen and thereby impede the reduction of the corrosion product. 
Complexing agents can increase current efficiency a great deal [2], even in solutions with 
a high pH. 

4) Temperature--lowering the temperature of the electrolyte used for electrochemical 
reduction decreases the rate of autoreduction [4] and thereby improves current efficiency. 



5. Possible Applications 

Two final suggestions need to be made with regard to possible applications of electro- 
chemical reduction to the study of metal artifacts other than the usual one of removing 
corrosion products for restoration purposes. These suggestions are rather speculative and 
need to be thoroughly researched before they can be applied, but they are worth mentioning. 

1) Establishing the similarity between corrosion products on different artifacts--it 
would be useful to know if the corrosion product on an artifact A is the same as the 



64 



corrosion product on artifact B even though they both exist on the same kind of metal, 
were found together, but may have corrosion products produced under different conditions. 
An attractive possibility which should be backed up with a great deal of research, is to 
use the sort of information shown in figure 2, the position and width of reduction pla- 
teaus, to help establish similarities or differences between corrosion products. For 
example. Hoar and Stockbridge [5] were able to measure the proportion of oxides to sul- 
fides present in films on copper using reduction plateaus. 

2) Determination of age of corrosion products--Sato and Cohen [6] have examined 
oxides on iron aged for different lengths of time. They related the amount of charge to 
reduce the oxide to the aging time, and found differences in reduction times for different 
aging periods. Their aging periods were extremely short (5000 minutes ^ 3.5 days) in 
archaeological terms but perhaps with sufficient research such an approach may prove to be 
a useful tool for artifact examination. 

The above examples serve to emphasize that electrochemical reduction has the poten- 
tialities to serve as a tool for gaining valuable information about an artifact in addi- 
tion to its main use for the conservator, the removal of corrosion products for resto- 
ration purposes. 



References 

[1] Greene, N. D. , Moebus, G. A., and Baldwin, M. H., Corrosion, 29, 234 (1973). 

[2] Oswin, H. G. and Cohen, M., J. Eleatrochem. Soo. 104, 9 (1957). 

[3] Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions (London, 
Pergamon Press, (1966)) pp. 76-79. 

[4] Pryor, M. J. and Evans, V. R., J. Chem. Soo., 1950, 1259, 1266 (1950). 

[5] Hoar, T. P. and Stockbridge, C. D., Eleetrochemioa Acta, 3, 94 (1960). 

[6] Sato, N. and Cohen, M., J". Eleatrochem. Soo. rn_, 624 (1964). 

See also: 

Campbell, W. E. and Thomas, U. B., Trans. Eleatrochem. Soo. 91_, 623 (1947). 
Allen, J. A., Trans. Faraday Soa. 43, 273 (1952). 

Davies, D. E., Evans, U. R. , and Agar, S. N., Proa. Roy. Soa. (a) 225, 443 (1954). 
Stockbridge, C. D., Sewell, P. B., and Cohen, M., J. Eleatrochem. Soa. 106, 928 (1961). 



65 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



MEASURES FOR PREVENTING CORROSION OF METALS 



R. T. Foley 

Department of Chemistry 
The American University 
Washington, D.C. 20016 



The objective of this paper is to summarize some of the methods that have been used 
to protect metals against environmental corrosion with particular reference to museum 
articles. 

Traditionally, methods of protection have been based, more or less, on the definition 
of corrosion, i.e., the interaction of a metal with its environment leading to deteri- 
oration of the metal. Accepting such a definition, it then appears logical to protect a 
metal by: 

a) treating the metal to render it corrosion resistant, or 

b) treating the environment to render it non-corrosive, or 

c) separating the two. 



1. Some Restrictions 

Because we are dealing with the corrosion of metal artifacts, things of interest to 
museums, it would appear that certain measures of great usefulness in corrosion technology 
would be non-applicable to the present problem. Specifically, reference is made to hot- 
metallizing processes, such as galvanizing and aluminizing, wherein the metal structure is 
immersed in molten zinc or molten aluminum and the surface is reacted and converted to a 
zinc or aluminum alloy. Another method of doubtful value might be alloying, although some 
mention is made of tarnish-resistant silver alloys later because of the wide and conti- 
nuing interest in this subject. But the use of alloys as a protection measure is limited 
because, first, the alloy for the metal artifact might have been selected several centuries 
ago, and secondly, if the corrosion scientist were to attempt to specify that the artist 
work not with bronze or iron but, rather, with stainless steel, Hastelloy C, or titanium, 
this suggestion, I suspect, would be il 1 -received. 

Also, the second method, the treatment of the environment, might pose some problem 
because, at the start, we do not always know what the environment will be. It should be 
possible to control the atmosphere in a museum case, and the example of the control of 
humidity in the preservation of bronzes at the Freer Gallery is cited. But, it would be 
much more difficult to control the environment in the rotunda of the U.S. Capitol. 

In spite of certain restrictions, there are useful protective measures that can be 
considered in the preservation of metal artifacts. 



2. Protection by Natural Corrosion Products 

It should be recognized that, whereas the environment is often considered as the 
ultimate adversary in the destruction of metal objects, this destructive capability is 
more often due, not to the natural environment but more to foreign objects which we propel 
into the environment, such as soot and sulfur dioxide molecules. The atmosphere can be 
effective in developing its own natural means of protection and a few cases may be cited 
of the development of passive films in nature and how the electrochemist would use the 
same principle to achieve protection. 



67 



Iron, for example, which usually rusts at an excessive rate, in certain environments 
will develop a passive film of oxide, probably 10 to 25 A thick. Iron statues have been 
known to exist for centuries in relatively uncontaminated atmospheres. One of the clas- 
sical cases is the Iron Pillar at Old Delphi, India, pictured in figure 1. The Iron 
Pillar is a shaft, about 24 feet high, of inscribed wrought iron, erected in 400 A.D. and 
still preserved [1]-^. 




Figure 1. The Iron Pillar of Mehrauli, near Delhi, erected 
about 400 A,D., and showing no signs of rusting. 

The green patina formed on copper roofs in some cases offers protection for many 
years. The determination of the chemical composition of this green patina was one of the 
first systematic corrosion studies ever made. This investigation was conducted in England 
by Vernon and his associates [2,3] and involved collecting and analyzing the corrosion 
products from roofs that had been exposed to the weather anywhere from 12 to 300 years. 
The composition of the green patina varied considerably depending on the location of the 
structure and the approximate composition varied with the atmosphere as shown in table 1. 
The figures do not add up to 100 percent because there were many other minor constituents 
in the film. Also, it should not be inferred that CuS0i+ exists as a distinct and separate 
compound. These compounds were found as double salts since many of these copper compounds 
are isomorphic. Several general conclusions were drawn from the study. A green patina 
was present in all cases of considerable age although a brown film was formed at first. 
The copper sulfate compound was usually predominant, with the copper carbonate present in 
low concentration. Where industrial pollution is low in the urban-marine environment, 
CUCI2 may predominate over CuSO^. The composition of these naturally formed corrosion 
product films was well summarized by Leidheiser in his monograph on the corrosion of 
copper alloys [4]. 

The observation of such cases as these has led the corrosion scientist to attempt to 
reproduce the behavior of Nature and protect metals by the formation of anodic films. The 
principle is illustrated in figure 2. As an anodic current is first applied, the iron 
electrode tends to dissolve. At a critical potential, passivation sets in and the corro- 
sion current drops to a very low value. In this range of anodic passivity the active 
metal is essentially corrosion resistant. A method based on this principle has been used 
to protect ferrous alloys from corrosion in concentrated acids [5]. The anodizing of 
aluminum in sulfuric acid solutions is another example of the application of^this principle. 
Aluminum forms a naturally protective oxide film of a thickness of about 25 A thick. 



^Figures in brackets indicate the literature references at the end of this paper. 



68 



Table 1. Composition of green patina from different locations. 



Location of 
structure 



Age of 
Structure (years) 



Composition of 



Urban 


30 


CuSOi, (49.8) 
CuCOs (14.6) 
Cu(0H)2(9.6) 


Rural 


300 


CuSOi. (25.6) 
CuCOs (1.4) 
Cu(0H)2(58.5) 


Marine 


13 


CuSOi, (2.5) 
CuCOs (12.8) 
CuCl2 (26.7) 
Cu(0H)2 (52.5) 


Urban-marine 


38 


CUSO4 (29.7) 
CUCI2 (4.6) 
Cu(0H)2 (61.5) 



2.0 




Figure 2. The protection of iron by anodic 
polarization. 



■2.0 -1.0 0.0 1.0 

Log current density 



2.0 



Relatively thick anodized films are produced by electrolysis in 5 percent sulfuric acid 
and "sealing" in hot water. So-called "non-porous" films are produced by electrolysis in 
tartrate solutions. The art of anodizing is well-founded and procedures have been worked 
out to develop corrosion resistant oxide films on aluminum objects without loss of def- 
inition. Some procedures of coating metals with hydrous oxides, a closely related process, 
are discussed below. 



3. Inhibitors 

The employment of inhibitors may involve, in one case, the altering of the environment, 
but usually involves separating the metal from the environment. There are cases, for 



69 



example the addition of alkali to an acid solution, which merely serve to neutralize the 
agent responsible for metal dissolution. More common, however, are the inhibitors that 
function through the adsorption of molecules on the metal surface. This subject has been 
appraised by Foroulis in his excellent review paper on inhibitors [6]. 

Inhibitors might be either inorganic or organic compounds. Examples of inorganic 
inhibitors are chromates, molybdates, phosphates, silicates, nitrites and tungstates. 
These compounds are usually effective in low concentrations, in the range of 100 to 1000 
ppm by weight. Organic inhibitors cover a very large number of organic compounds which 
may be characterized [6] by the type of bonding which is achieved with the metal. Certain 
compounds, such as nitrogen-containing compounds as aniline, butylamine, and pyridine, are 
thought to function by electrostatic attraction as does benzoic acid. A second type of 
compound is thought to adsorb and form a chemi sorption bond. Some sulfur compounds and 
possibly some amines are chemisorbed. Benzotriazol e and tolytriazol e, both effective 
inhibitors of copper alloy corrosion, are believed to operate through chemisorption 
[7,8]. A further type of bond is thought to operate through pi bond interaction with the 
corroding metal. The so-called pi bonds exist in unsaturated compounds and the electrons 
forming the bonds instead of being localized, i.e., tied to specific carbon atoms, float 
over the whole molecule. Thus, table 2, taken from the work of Foster et al. [9], shows 
that several compounds of very similar composition but varying degrees of saturation are 
dramatically different with respect to their effectiveness as inhibitors. 



Table 2. Corrosion inhibition data with 1020 carbon 



steel in 2.8 


N HCl at 65 °C. 


Inhibitor (0.4 wt %) 


Corrosion rate 
(metal loss, mg/cm^/d) 


Blank 


> 48,900 


CH3-CH2-CH2-OH 


> 48,900 


CH2=CH-CH2-0H 


13,200 


HCHC-CH2-OH 


146 


,CH3 
H-ChC-C-OH 

CH2~CH3 


1,956 



These inhibitors function to some extent in providing a physical barrier separating 
the metal from the environment. But it is important to note that monomol ecul ar films are 
effective and that, in certain cases, fractions of monolayers reduce the corrosion rate to 
very low values suggesting that the compound, instead of forming an impervious barrier, 
acts by blocking selective sites that would catalyze specific reactions, as the reduction 
of hydrogen, which are key steps in the multi-step corrosion process. 



4. Treating the Environment- Vapor Phase Inhibitors 

The most straightforward way to protect metals by the treatment of the environment is 
through the reduction in the water content necessary for aqueous corrosion mechanisms. It 
has been found that if the relative humidity of the atmosphere is below about 60 percent, 
such normally detrimental atmospheric pollutants as sulfates and chlorides do not corrode 
iron, copper, zinc, or aluminum. Therefore, the practical goal has been to keep the 
relative humidity below 50 percent. This can be achieved by using desiccants of which 
silica gel and activated alumina are among the easiest to use [lOj. Moreover, these 
materials may be regenerated by heating: silica gel to 130-300 °C and activated alumina 
to 150-170 °C. Formulas have been worked out giving the amount (weight) of the desiccant 
in terms of the volume of air to be treated, the surface area of the metal, the type of 
barrier, and other factors. 



70 



A more positive method involves vapor phase inhibitors. These are compounds with a 
significant vapor pressure so that in a closed area they will volatilize and later condense 
on the metallic surface. The compound contains functional groups, nitrites, or amines 
that, when present in the condensed moisture film, will inhibit corrosion. The two 
original vapor phase inhibitors were dicyclohexylamine nitrite and cyclohexylamine carbonate 
[11,12]. The dicyclohexylamine nitrite has a vapor pressure of 0.0002 mm Hg at 25 °C and 
a solubility of 3.9 g in 100 g H2O which gives a solution of a pH of about 6.8. The 
carbonate has a higher vapor pressure, 0.4 mm Hg at 25 °C, a solubility of 55 g in 100 g 
of H2O, and produces a solution with a pH of 10.2. These compounds have been used to 
protect ferrous materials and may attack certain non-ferrous materials such as lead and 
cadmium. On the other hand, these compounds offer excellent protection for aluminum. In 
principle, there appears to be no reason why specific vapor phase inhibitors should not be 
developed for the protection of copper, brass, tin, lead, or any other alloy. 

These vapor phase inhibitors have been incorporated into papers with which machined 
parts, for example, may be wrapped and stored for a period of years. One volatile corrosion 
inhibitor is marketed by Shell under the name VPI 260. 



5. Protection by Electroplating 

One intriguing question in science is how far back metal coating goes. Or, more 
specifically, when did artisans first use electroplating to protect or ornament baser 
metals. In the thinking of contemporary el ectrochemi sts this cannot go back before 1800 
when Volta constructed his battery. In 1806, Davy electroplated sodium and potassium from 
their hydroxides, in the 1820's and 1830's Faraday did his important experiments on elec- 
trolysis, and only in the 1850-60 period were practical cyanide baths described in the 
literature. So, in terms of our more recent records, we would say that electroplating for 
the protection of metals does not go back much over 100 years. On the other hand, one 
might make a case for the existence of a certain amount of electroplating art approximately 
2000 years ago^. 




copper disc in dimensions to those unearthed by Konig. 



^In the presentation of this paper at the Conference on March 17, 1976, the Gundestrup 
Cauldron was cited as a silver-plated metal artifact dating from the first century B.C. 
This observation was supported by reference to MacCana's monograph [13] wherein the 
Cauldron is described in great detail. More detailed investigation and correspondence 
with people who have studied the original Cauldron at the Danish National Museum in 
Copenhagen definitely indicate that it is constructed entirely of silver and not silver 
plated. The Author suspects that the difficulty may have come from translation. In the 
booklet, Gundestrup Kedelen by 01 e Kl i ndt-Jensen , the Cauldron is described as "an embossed 
silver-gilt vessel," meaning a silver vessel partially gilded with gold, rather than a 
vessel gilded with silver. 

71 



In 1935, the German archeol ogi st, Wilhelm Konig, was excavating near Bagdad and found 
the remains of a Parthian town [14]. The Parthians dominated this region in the period 250 
B.C. to 224 A.D. Konig found the remains of what has been concluded to be copper-iron 
batteries. Ceramic vases, copper cylinders, iron rods, asphalt stoppers, and connecting 
wires were found on the site. Similar pieces were assembled into a working battery using 
a copper sulfate electrolyte and this battery is depicted in figure 3. Other electrolytes, 
such as acetic acid and citric acid, were known to people in those days. A large number of 
these vases and parts were found and it is easy to visualize a series assembly of several 
batteries to produce a reasonable voltage. With this evidence one must leave open the 
possibility of electrodeposition being available for metal protection in earlier times. 

In modern technology the silver plating of copper and copper alloys is well known. 
The more recent effort has been directed toward the development of plating baths that would 
yield non-tarnishing or non-corroding deposits. A recent U.S. Patent [15] describes a Ag- 
Sn-Ni alloy containing 70 percent Ag, 26 percent Sn, and 4 percent Ni which has the appear- 
ance of Ag but excellent tarnish resistance. This alloy was plated from a bath with the 
following composition and under conditions indicated: 



Silver cyanide 0.65 g/1 

Tin hydroxide 5.3 g/1 

Nickel hydroxide 0.13 g/1 

Sodium hydroxide 15 g/1 

Sodium cyanide 100 g/1 

Temperature 120-150 °F 
Current density 15-45 A/ft2 

Potential 1-1.5 V 



The composition of these plating baths is usually arrived at in an empirical manner as 
the deposition of each element is controlled by its particular potential -current density 
rel ationship. 

A recent Japanese patent [16] describes a procedure by which a tarnish resistant alloy 
is prepared by electrodeposition followed by thermal diffusion. The brass object is plated 
with 12 ym of silver, 0.2 ym of zinc, 0.3 ym of indium, and 1 ym of tin, in that order. 
The electroplated brass is heated in argon at 750 °C for 15 minutes, and slowly cooled. 
The alloy then has a composition of 88.1 percent Ag, 8.2 percent Sn, 2.2 percent In, and 
1 . 5 percent Zn . 



6. Protection with Hydrous-Oxides 



There are a number of reports in the literature describing the production of non- 
tarnishing surfaces by hydrous oxides. These oxides have been deposited el ectrophoretical ly , 
by immersion treatments wherein a salt was hydrolyzed, or by selective oxidation of an 
oxidizable metal in an alloy. Path et at. [17] described the protection of silver from 
tarnishing by the el ectrophoretic deposition of alumina from a bath containing aluminum 
sulfate and ammonium oxalate. Burkhardt [18] used a solution of 1.5 percent tin chloride 
in anhydrous methanol and deposited a stannic oxide solution by hydrolysis of the chloride. 
In some work done in our own laboratory with The Silver Institute it was found that the 
combination of these two techniques was successful in rendering silver relatively free of 
tarnish. The codeposition of Mg, Be, or Al with silver is described by Saifullin et al. 
[19], but it is not clear whether the metal or some compound is codeposited. From the 
conditions of the experiments, one would conclude that a metal hydroxide was deposited. 
The electrolyte and conditions used by Saifullin and coworkers were as follows: 



AgCl (freshly precipitated) 40 

K4Fe(CN)5-3H20 200 

K2CO3 30 

pH 9.4 

Current density (A/dm2) 0.1-1.0 

Additive (NgSO^-ZHjO) 0.001-0.1 g eq/1 

Silver plated from baths with 0.01 g eq/1 MgSO^ as an additive is claimed to be greatly 
resistant to tarnish. 



72 



Another oxide treatment is described by Yamazaki [20]. The silver surface is treated 
cathodically in a solution containing SnClj (20 g/1 ) and HCl (50 ml/1). A film of tin 
oxide or hydroxide is formed in 3 minutes at a current density of 0.02 amp/cm2. Alternate 
electrolytes are tin borofluoride and tin fluorosil icate. 

A further oxide treatment [21], this one depositing beryllium oxide, is used to protect 
silver, copper, or brass against tarnishing. This bath contains 0.7 g/1 of beryllium as a 
beryllium salt and the pH of the bath is maintained between 4.5 to 6.2. 



7. Vacuum Deposition of Metals 

The vacuum deposition of metals offers a method with several apparent advantages. The 
surface preparation need not be as stringent as required for el ectrodeposi tion and the 
details of the surface will be faithfully followed. Metals such as platinum, rhodium, and 
iridium may be deposited [22]. Certain metals such as chromium may be deposited in vacuum 
from the decomposition of a salt, e.g., chromous or chromic iodide. Some metals, such as 
aluminum are deposited in large-scale commercial operations on both metallic and non- 
metallic substrates. 



8. The Silver Tarnishing Problem 

Over the years one of the problems that has received a tremendous amount of attention 
is the prevention of tarnishing of silver. Special attention should be given to this 
problem as virtually every method known has been explored, most in great depth. 

A large number of silver alloys have been made and tested in sulfur atmospheres, in 
fact, alloys incorporating every metal with appreciable solubility in silver have been 
tested. Many investigations have been restricted to an upper limit of 7.5 percent alloying 
element to qualify as a sterling silver composition. Another specification is appearance. 
The alloy should look like silver, e.g., not have a gold or steely cast. In a typical 
investigation [23] a large number of alloys was prepared and tested in a sulfur atmosphere. 
Alloys containing 15 percent zinc, 20 percent cadmium, and 10 percent indium were highly 
resistant to tarnishing. In this investigation a series of silver-copper alloys were 
prepared--recal 1 the composition of sterling silver is 92.5 percent silver and 7.5 percent 
copper. All of the Ag-Cu alloys tarnished at a more rapid rate than Ag itself. In fact, 
of all the silver alloys available, the silver-copper represents the poorest possible 
selection for a tarnish-resistant composition. For tarnish resistance alone, a very good 
alloy would be one with a composition of 8-10 percent indium. With that result in mind, a 
number of years ago the Indium Corporation of America made up a number of sets of silver 
services with a tarnish resistant silver-indium alloy replacing the sterling composition. 
This development was never accepted commercially. Although quite resistant to tarnish, it 
had a slightly steely appearance, i.e., it did not look exactly like sterling silver. 

One of the most successful tarnish resistance treatments has been the use of electro- 
lytically deposited coatings, such as those produced by depositing alumina or beryllia on 
the surface. It should be noted that rarely, if ever, does a coating render silver tarnish 
proof. The coating may give to the silver surface considerable resistance but, in a matter 
of time, it will tarnish. One objection to the electrolytic deposits is a loss of reflec- 
tance of a highly polished surface. This might not be apparent to the layman but is 
readily evident to the expert at the Franklin Mint as he examines his proof coin. 

Certain inhibitors are effective in postponing the onset of tarnishing of silver. It 
is reported that silver plate, if treated with the phosphoric acid ester of a hydroxy fatty 
acid, tarnishes less readily. The silver article is immersed in a 3 percent solution of 
the ester dissolved in trichloroethane and then dried at room temperature. 

A proprietary solution claimed to produce an anti-tarnish film includes thiourea and 
phosphoric acid. 

Silver has been successfully protected against tarnish and corrosion by extremely thin 
coatings of rhodium, e.g., 0.0001 to 0.0002 inch thick. Such coatings insure that the 
silver surface will not suffer a loss in reflectivity which is a critical item in many 

73 



silver applications. Finally, to prevent tarnishing of silver, and copper as well, a 
technique [25] has been employed which uses air-filter pads impregnated with finely divided 
Cu , Pb, or Zn hydroxide to absorb and react with the sulfide. 



9, Acknowledgement 

The Author would like to acknowledge the considerable help furnished by Mr. Richard L. 
Davies, Executive Director of The Silver Institute, in assembling the literature on silver 
tarnishing. Dr. Sarada furnished the photograph of the Iron Pillar in Delphi. 

References 

1] Basham, A. L., The Wonder that was India, pp. 218-219 (Sidgwick and Jackson, London, 
1954). 

2] Vernon, W. H. J. and Whitby, L., J. Inst. Metals, 42, 181 (1929); 44, 389 (1930). 
3] Vernon, W. H. J., J. Inst. Metals, 49, 153 (1932). 

4] Leidheiser, H., Jr., The Corrosion of Copper, Tin and Their Alloys (John Wiley and 
Sons, New York, 1971). 

5] Sudbury, J., Riggs, 0., and Shock, D., Corrosion, 16, 47t (1960). 

6] Foroulis, Z. A., Molecular Designing of Organic Corrosion Inhibitors, in Symposium on 
The Coupling of Basic and Applied Corrosion Research, NACE, p. 24, 1969. 

7] Dugdale, I. and Cotton, J. B., Corrosion Science, 3_, 69 (1963). 

8] Mansfeld, F. and Smith, T., Corrosion, 29, 105 (1973). 

9] Foster, G. L., Oakes, B. D., and Kuceda, C. H., Ind. Eng. Chem. , ]_, 825 C1 959). 
10 



Shreir, L. L., ed., Corrosion Control, vol. 2, p. 18.3 (New York, John Wiley and Sons, 
1963). 

Wachter, A., Skei , T., and Stillman, N., Corrosion, ]_, 284t (1951}. 

Stroud, E. G. and Vernon, W. H. J., J. Appl. Chem., 2, 178 (1952). 

MacCana, P., Celtic Mythology , (Hamlyn Publishing Group, London, 1975). 

Schwalb, H. M., Electric Batteries of 2000 Years Ago, Science Digest, p. 17, April 
1957. 

Viglione, G. T., U.S. Patent 3,778,259, Dec. 11, 1973. 
Kasai, K., Japanese Kokai Patent 73 66,037, Sept. 11, 1973. 

Fath, R., Bon, G., and Hasko, F., Vortrage des II. Gal vanotechnischen Symposiums, 
Budapest, Dec. 1966. 

Burkhardt, A., Metall. , 15, 344 (1961 ). 

Saifullin, R. S., Zaitseva, L. V., and Andreev, I. N., Zashchita Metallov, 8, 497 
(1972) . 

Yamazaki, Y., Japanese Patent 73 39,355, Nov. 22, 1973. 

Numa, T., Kimura, T., and Takano, M., Japanese Patent 74 01,984, Jan. 17, 1974. 



74 



[22] Powell, C. F., Campbell, I. E. and Gonser, B. W., Vapor Plating, (John Wiley and Sons, 
London, 1955). 

[23] Foley, R. T., Bolton, M. J., and Morrill, W., j. Eleotvoahem. Soa., 100, 538 (1953). 
[24] Ryu, K., Japanese Kodai Patent 74 94,941, Sept. 13, 1974. 

[25] Shreir, L. L., ed.. Corrosion Control, vol. 2, p. 18.9 (John Wiley and Sons, New York, 
1963). 



Discussion 

Phoebe Weil: Concerning the protective properties of basic copper sulfate on outdoor 
copper and bronze, we and others (for example Reiderer in Germany) have found that contrary 
to what has been observed in the case of copper roofing, where the green basic copper 
sulfate formed is highly but not totally protective of the metal beneath, that this same 
corrosion product on outdoor bronzes is not protective. The explanation is thought to 
arise from the differences in metallic structure (wrought in the case of roofing vs. cast 
for statues) and perhaps the greater likelihood in the case of statues for the presence of 
impurities and rougher surface finish {e.g., filed). We have observed surface attack on 
statues approximately an order of magnitude or more greater than that reported for wrought 
specimens exposed in atmospheric exposure tests in similar urban environments. 

R. T. Foley: Thank you. 

S. K. Cobum: To confirm the use of vapor phase inhibitors, a revolution in packaging 
steel is taking place wherein a large barge load of coils of unwrapped steel are being 
protected by fogging the confined area with a vapor phase inhibitor and transporting the 
barge through areas where temperatures can result in condensation of moisture. The exper- 
imental results have been encouraging enough for us to send box cars and trucks loaded with 
unwrapped carbon steel to customers. Of course, the recipient, likewise, has to find an 
area in his plant where he can cover the steel with some sort of a shroud and fog the air 
space within the shroud to continue the protective atmosphere. 

Experiments of this sort are continuing in different parts of the country where a 
variety of daily temperature differences characterize the respective locations. 

R. T. Foley: Thank you. That was a very useful comment. 

M. Pourbaix: You have shown a slide of a silver plated copper bowl that has been found 
buried in the soil in Denmark and appeared to be in exceptionally good condition. Is 
anything known about the corrosivity of the soil where the bowl was found? 

R. T. Foley: No. I am sorry but I have no information about the soil. 

K. E. Holm: This is a correction to what has just been said about the Gundestrup cauldron. 

This cauldron, which is one of the most famous objects of the National Museum of 
Denmark, is made essentially of silver and is not manufactured out of silver plated copper. 

Several authors have written about the cauldron, the find of which was originally 
published by Sophus Muller in Nordiske Fortidsminder Vol. I, 1892 (with a summary in 
French). The analysis made then give the figures 97 percent silver and oa. 3 percent gold. 
The rest is unspecified. The silver plates have been soldered together with tin-solder. 
The cauldron is partly gilded with a gold foil which in some places does not adhere very 
wel 1 . 

F. Halahan: I wonder if the speaker could give any information on the use of mercaptans 
for the protection of silver and whether he knows if they are used in long-term polishes. 

R. T. Foley: Under most circumstances mercaptans will corrode silver in much the same 
manner as sulphur. However, thiourea, which of course is not a mercaptan but very close to 
it, and some compounds like benzotriazol are used. 

75 



F, Halahan: I thought that the Long Tarnish-shield polishes for silver had mercaptans in 
them. At least according to the patent in England. 

if. T. Foley: It is very possible that proprietary compositions do have mercaptans. 

R. M. Organ: Mercaptans are supposedly present in Goddard's "Tarnish Preventi ve--Not a 
Lacquer". Long chain mercaptans with 15-23 carbons in the chain. The tarnish inhibitor 
"SEL-REX" contains the same substance, nominally, and analysis by infrared spectrophotometry 
in CAL confirmed the presence of mercaptans. 

W. T. Chase: (Added in proof) Use of mercaptans as anti-tarnish agents incorporated into 
silver polish is covered in a U.S. Patent granted to James G. Murphy (U.S. Pat. 2,841,501; 
re-issue 24,819 (May 3, 1 960). He shows as examples n-hexodecane-1 -thiol , n-dodecane-1 - 
thiol, and other thiols. 



76 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gai thersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



A REVIEW OF THE HISTORY AND PRACTICE OF PATINATION 



Phoebe Dent Weil 

Research Associate and Conservator 
Center for Archaeometry 
Washington University 
St. Louis, Missouri 63130 



The word patina today is most commonly associated with the handsome green corrosion 
products found on certain ancient bronzes recovered after long burial in the soil, or with 
colors intentionally produced on bronze using various chemicals either with or without heat. 
My discussion will focus on the latter sort, commonly termed artifical patination. For a 
proper study of the history of artificial patination of metallic artifacts, particularly 
bronzes, one must consider as well the larger question of historical attitudes towards the 
interaction of color and form. Such a study serves several useful purposes: first and most 
important, it serves as a step toward achieving the objective that is the basis for all 
aesthetic decisions for connoisseur, curator, and conservator alike, namely that of attempting 
to see the object through the eyes of its maker or makers--of discovering the artist's 
original intent. What evidence do we have to call upon that will enable us to see antique 
bronzes through the eyes of the ancient artisans of the Berlin Foundry Vase, (fig. 1), here 
shown putting the final polish and burnish on a colossal bronze sculpture; or the sculptor 




Figure 1. Berlin Foundry Cup, detail showing 
polishing and burnishing of a bronze statue 
[Corpus Vasorum, Berlin, II). 



Foggini» here seen (fig. 2) in his studio around the year 1700, beginning the first clay 
sketch-model or hozzetto that will later be perfected, enlarged, and translated into bronze? 
Further, such an inquiry will assist us in distinguishing as the ancients did, between 
aerugo nobilis or "noble patina" and virue aerugo or patina that is destructive either 



77 



Figure 2. Foggini at work (Lankheit, Floren- 
tinisdher Baroakplastik, Munich, 1962, Fig. 
IV, caracature by A. D. Gabbiani, Florence, 
Uffizi). 




visually or physically or both; and will provide a basis for deciding what, if any, treat- 
ment will most appropriately bring the object closest to its originally-intended appearance. 

The problem of treating metallic objects is complicated by the fact that patina, i.e., 
corrosion, is formed at the expense of the substance of the object itself, even occasionally 
to the point of complete mineralization, and once severe alteration or corrosion has occurred 
it is impossible to determine original coloration or finish from physical evidence. Further, 
these corrosion products and accretions can in themselves contain highly important historical 
information, whether they be the corrosion products and layers of soot and dust on an urban 
bronze that can perhaps reveal valuable information about past environmental history, or, 
in buried artifacts, a bit of soil that can help to identify provenance and establish 
authenticity. 

This dilemma, the problems of determining original appearance, of identifying changes 
that have occurred during the course of time, and of judgment in distinguishing disfiguring 
foreign material from authentic substance of the artifact or work of art are among the most 
keenly debated problems in Art Conservation. The most publicized articulation of this 
dilemma that we might call "The Patina Dilemma" came about as a result of the so-called 
"Cleaning Controversy" in London, with articles published in the Burlington Magazine 
between 1949 and 1962 [1]^. The Cleaning Controversy related not to metallic artifacts but 
to paintings, for which the changes wrought by time have beem summarily identified as 
patina. The arguments begin with rather polarized points of view of the Aesthetician versus 
the Scientist, the former speaking against "the raw and brutal surface laid bare by cleaning" 
and presenting a definition of patina for painting in terms of glazes or tinted varnishes 
claimed to have been purposely added by artists throughout the centuries to "tone down 
brightness" [2]. The Scientists held that treatment could be based on a purely scientific 
distinction between genuine patina or the alterations wrought by time on the authentic 
substance of the painting and other- disfigurements wrought by discoloration of once water- 
clear varnishes, overpaint and restoration and accretions of various sorts [3]. These 
thesis-antithesis viewpoints were brought to rather elegantly stated synthesis in the articles 
by Stephen Rees-Jones and Denis Mahon who stressed the absolute necessity, to quote Mahon, 



Figures in brackets indicate the literature references at the end of this paper. 

78 



of "the genuine pooling, without distrust and awisre-pensee of the most diverse forms of 
knowledge" [4]. Rees-Jones stressed the neaesaity for interplay between the methods of the 
laboratory and aesthetic and historic criteria on the question of patina, and quotes Paul 
Coremans in emphasizing the fact that chemical analysis and physical method, "should be 
viewed in relation to one another, and in the light of historical, aesthetic, stylistic and 
technical data to be derived from the curator's, aesthetician's, and restorer's examination 
of a picture" [5], 

It is surely this total-view that we must ideally bring to bear on our approach to the 
consideration of patina on metallic objects as well. Perhaps in no other area of connois- 
seurship is one's judgment of beauty so dependent upon scientific understanding, for, as we 
often see, one man's "handsome patina" can be another man's "ugly corrosion products." The 
aesthetician will find that an acquaintance with the intellectual processes necessary to 
understand structure and cause and effect, instead of interfering with his sensory reaction, 
will rather enhance it; and the scientist will become more acutely aware of the demands made 
by historic and aesthetic criteria. 

Turning now more specifically to our concern with the coloration of bronze, let us 
begin with a consideration of the material itself. Most of us are familiar with the bright, 
salmon-colored appearance of the highly-polished metal, as well as with the dark, red-brown 
of the oxidized surface of the metal. It is precisely the difference between the appearance 
of a new penny and that of an old penny. Now something different happens when bronze is 
exposed to increased amounts of moisture and various chemical substances, either gaseous or 
solid, that will react with the metal. Chemical reactions will occur that will tend to 
bring the metal into equilibrium with its ambient condition whether above or below ground, 
inside or out. Moisture is the essential ingredient for further change to take place, and 
if moisture could be excluded no further chemical changes other than the formation of the 
thin coating of red-brown oxide would occur. Once moisture becomes accessible to the bronze 
artifact, what happens is far from simple and still not entirely understood. Lewin and 
Alexander, in the introduction to their annotated bibliography on the composition and struc- 
ture of natural patinas, published in Art and Archaeology Teahniaal Abstracts [6], point 
out the considerable complexity of the chemical systems, as an example of which they mention 
that there are at least eight sulfate-containing copper compounds that can form when copper 
alloys corrode in the presence of sulfur compounds, and that some twenty-five copper-containing 
minerals have been identified in natural patinas. Furthermore, ambient conditions can 
change, as it must be remembered that prior to the Industrial Revolution in the 19th century, 
outdoor bronzes developed natural patinas described as attractive, thin, compact, translucent, 
generally red-brown and more or less tinged with green depending upon accessibility of 
moisture [7]. This stands in contrast to the mottled green and black appearance of modern 
urban bronzes (fig. 3) whose surfaces have been attacked by various components of urban air, 
particularly sulfur compounds. 



Figure 3. George Washington, 
LaFayette Square, St. Louis: 
a typically disfigured outdoor 
bronze. 




79 



r 



Thus, when we read Webster's current definition of patina as: "1. a fine crust or film 
on bronze or copper, usually green or greenish blue, formed by natural oxidation and often 
valued as being ornamental," and "2. any thin coating or color change resulting from age, as 
on old wood or silver" [8], we have a definition that, rather than being definitive, contains 
all of the elements that contribute towards present confusion on the subject: 

1. "Natural oxidation" is an imprecise way to describe the extraordinarily complex 
variety of chemical reactions, of which oxidation is only one, that can occur on a metallic 
bronze surface as it works its way toward equilibrium with its environment. 

2. The results are only rarely a "fine crust or film," but more often range from thin 
to thick, warty, and multi-layered; from compact to porous; from finely-crystalline to 
containing crystals of large size; smooth and enamel-like or rough and varied in texture; 
containing one or two simple compounds to a large number; and in colors spanning the entire 
spectrum: reds, grays, blacks, greens, blues, browns, purples, yellows, and oranges. 

If patina is valued as an indicator of age, when, if ever should it be removed? 

What are the criteria for the ornamental qualities of patina? 

A revealing way to begin a study of historical attitudes towards patination is by 
having a look at the origins and use of the word itself. Most dictionaries give the origin 
of the word as deriving from the Latin word for plate or pan [patena) [9], though it is 
often mentioned that the connection is uncertain. The key is perhaps to be found in the 
first known printed use of the word which occurs in Filippo Baldinucci's Art Dictionary of 
1681 where patena is defined, without reference to corrosion products of metals, as "a term 
used by painters, called by others a skin, namely that general dark tone which time causes 
to appear on paintings, that can occasionally be flattering to them" [10]. William Hogarth's 
depiction of Time Smoking a picture provides a good illustration of Baldinucci's definition 
(fig. 4). 




■ I ^/.^,r„ .y '. //,i;.,.r. lmr/<i/^/., . ^ .1. 



Figure 4. William Hogarth, "Time Smoking a Picture" 
(George, Hogarth to Cruikshank: Social Change in 
Grapic Satire , fig. 10). 



80 



The most likely origin of the word patina is the old Italian word patena used to refer 
to a shiny dark varnish applied to shoes [11]. Baldinucci's use of the term indicates 
that it came to refer generally to the effects of time and only later was applied to such 
effects on metallic objects. The first appearance of the word in a dictionary with the 
definition referring to green corrosion products on bronze occurs in the French Enayalopedie 
of 1751, where patine is defined as follows: "There is no French word to express that 
beautiful and brilliant color of verdigris that copper does not always assume; the attrac- 
tiveness of this color to the eye and the difficulty in describing it (because all coppers 
do not uniformly develop it) is highly valued by the Italians who call it patina as one 
dares to do here after their example and by the example of M. le Comte de Caylus who states 
correctly that one should be allowed to adopt a foreign word at least in the language of the 
arts of which this Encyclopedia is the Dictionary" [12]. Webster would have done well to 
emulate French caution. 

The term patina used to refer to green corrosion products on bronzes was most likely 
stimulated by the expanding and heightened interest in archaeology in the 18th century. 
According to the Oxford English Diationary, the earliest recorded English usage of patina to 
describe green corrosion products on bronze is as late as 1797. According to the Italian 
etymological dictionaries, the Italian verbs meaning to give a patina, patinated, and 
patinator are all 19th century in origin [13]. 

The philological aspects of the word appear to reflect the historical situation: while 
coloristic effects were achieved in bronze sculpture and other metallic artifacts by a 
variety of means from earliest times, it was not until the 19th century that artificial 
pati nation of bronzes by chemical means, with or without heat, was generally and widely 
practiced. 

Observation of, indeed a fascination with the colored corrosion products of copper and 
copper alloys dates back to ancient times. Experimental artificial production of these 
substances from copper and other metals was perhaps even fundamental to the development of 
alchemy and thereby of chemistry itself [14]. The most convincing studies regarding the 
problem of patination of bronzes in ancient art have been made by Erich Pernice in 1910 
[15], followed by the discussion by Gisela Richter in the 1915 catalogue of the ancient 
bronzes in the Metropolitan Museum [16], and a more recent study by H. Otto in 1959 [17] 
based on x-ray diffraction analysis of patinas. Both Pernice and Richter provide rich 
documentation from ancient literature and inscriptions as well as from the objects themselves, 
and Pernice goes a further step in having himself recreated the ancient method of finishing 
bronze. Both Pernice and Richter' s work have been frequently overlooked in recent studies. 

The most important of the ancient literary sources on the subject of patination are to 
be found in Plutarch, the Greek biographer and historian, and the Roman, Pliny the Elder 
(Gaius Plinius Secundus), both of whose writings date from the first century A.D. Richter's 
account of the passage from Plutarch's De Fythiae oraauHs is worth quoting here: "A number 
of visitors to the sanctuary of Apollo at Delphi are made to discuss the question whether 
the patina on the bronze group in front of which they are standing is natural or artifical. 
One of them is admiring the beautiful surface of the bronze, which resembles neither dirt 
nor rust, but looks as if it had been dipped in a bath of brilliant blue color. 'I wonder,' 
he adds, 'whether the ancient masters used a certain mixture or preparation on their bronzes?' 
In the discussion that follows, various suggestions are made to explain the presence of the 
patina by physical conditions, for instance, that it is due to the action of the atmosphere 
which enters the bronze and forces out the rust (or rather corrosion); or that the bronze 
itself when it gets old exhales the rust." Miss Richter goes on to point out the great 
importance that Plutarch, "had no reason to believe in an artificial patina, but clearly 
decides in favor of a patina acquired by natural causes" [18]. 

To answer the question, "How did the ancients normally finish their statues?", both 
Pernice and Miss Richter marshall an impressive array of evidence supporting the thesis that 
the ancients finished their objects in bronze by giving them a painstakingly careful, high, 
lustrous polish. All seams and welds of the separately cast pieces were ingeniously and 
carefully hidden so that all exposed surfaces could be polished to perfection by specialists 
in the foundry. When one considers the great value of bronze metal in ancient times, it is 
not so surprising that the high lustre of the smoothed and polished metal itself was admired 
and judged the most desirable and normal method for finishing bronze artifacts of all kinds. 



81 



This is not to say that sculpture and other bronze artifacts were not without a variety of 
coloristic and textural effects, they were, but these were achieved by using alloys of 
contrasting colors, and other materials, such as ivory, stone or glass, as inlay or in 
juxtaposition, in combinations such as niello, silver and copper. Such contrasting effects 
were entirely dependent on a bright and untarnished appearance of the whole. The Delphi 
Charioteer has, for example, inset glass paste eyes and separately inserted bronze plate 
eyelids with eye lashes. This is consistent with the evidence we have for the appearance of 
Greek marble sculpture that was not only painted in bright colors, but often was fitted with 
accessories of different materials such as glass, ivory or metals of various sorts such as 
necklaces and earrings or diadems. And as the marble sculpture was provided with a protective 
coating of what was probably a wax-resin mixture (Vitruvius' Punic wax) [19], so was the 
bronze sculpture provided with a coating designed to exclude moisture and preserve the 
bright polish of the metal [20]. For both marbles and bronzes it was considered normal to 
remove and renew these coatings in order to maintain the original dazzling splendour of the 
sculptures. As Miss Richter pointed out in the passage from Plutarch, that it "follows 
indirectly that in his own time bronzes were kept in their natural finish; otherwise why 
should the Delphic visitors be surprised at the presence of a patina on Greek bronzes?" 
[21]. 

Several inscriptions survive from ancient times documenting provision for maintenance 
of bronzes. Pernice mentions an inscription from Chios of the fourth century B.C. in which 
the clerks of the market are told to see to it that a bronze statue of a tyrannicide be kept 
free from corrosion, and further the clerk is instructed to see that the statue be provided 
with a garland and kept bright [22]. 

The writings of Pliny [23] relevant to the finishing of bronze have been variously 
interpreted, but in the light of preceeding remarks he is found to be entirely consistent. 
The Latin term he uses to describe the green corrosion products of copper is aerugo, aeruginis 
that is to say the rust {robigine) of bronze (ass). Pliny speaks of a method of producing 
it artificially, by hanging copper metal in casks over strong vinegar [24]. The uses and 
purpose of producing this aerugo are primarily medicinal and cosmetic, such as for annointing 
eyes and the healing of sores and ulcers, and not in any way related to the finishing of 
statuary. When Pliny speaks of bronze and the varieties of bronze alloys he consistently 
speaks of the color of the metal itself, for example the addition of lead to Cyprus copper 
"produces the purple color seen in the bordered robes of statues" [25], and that certain 
bronzes had special value because of their color [26]. A particularly good case in point is 
Pliny's account of the statue of Athamas by Aristonidas: "When the artist Aristonidas 
desired to represent the madness of Athamas subsiding in repentence after he had hurled his 
son Learchus from the rock, he made a blend of copper and iron, in order that the blush of 
shame should be represented by rust of the iron shining through the brilliant surface of the 
bronze ( aeris relucente )" [27]. 

As to bronze treatments, Pliny mentions the use of vinegar and the urine of a child 
[urina pueri) for cleaning bronze [28]. For copper and bronze utensils he recommends 
frequent polishing and rubbing with oil or coating with vegetable pitch to preserve them 
from corrosion [29]. Perhaps the most frequently misinterpreted passage has to do with 
Pliny's statement about providing a protective coating for bronze statuary, the key words of 
which are: "bitumine antiqui tinguebant eas," which has frequently been interpreted to mean 
that the ancients painted their statues with a dark black coating of bitumen [30]. Con- 
sidering the care taken in inlay work and in contrasting colors of metals this would be 
unthinkably inappropriate. Pernice has demonstrated the correct interpretation of this 
passage based on experiments using pitch diluted with turpentine to brushing consistency 
which was then painted in a thin coating onto a piece of brightly polished bronze. "The 
wash," he claims, "increased rather than diminished the brightness of the bronze, and at the 
same time protected the surface from atmospheric effects" [31]. The entire passage in Pliny 
runs: "The ancients painted their statues with bitumen, which makes it more surprising that 
they afterward became fond of covering them with gold" [32], because, as Pernice has demon- 
strated, the bitumen coating gave the same appearance to the bronze as gilding, and therefore 
made gilding unnecessary. This is reminiscent of the discussion of ancient picture varnishes 
and the varnish of Apelles as described by Pliny that occurred during the London Cleaning 
Controversy. Those who supported the idea that the ancients applied a tinted varnish to 
obscure their bright colors pointed to the descriptions of the dark resin solutions apparently 
used [33]. Those arguing in favor of the ancients not having veiled their colors pointed to 
the fact that such natural resins most likely to have been used by the ancients, while 



82 



appearing miirky brown in a container, had the appearance of a shiny, essentially water- 
clear, transparent coating when applied to the painting [34]. 

We have even an inscription as late as 1076 A.D. on the bronze doors of the church of 
S. Michele at Monte S. Angelo in which the sponsor of the doors, Pantaleone d'Amalfi, 
instructs those in charge to clean the doors once a year so that they will always be shiny 
and bright [35]. 

So all evidence points to the fact that bronzes of all kinds, from artifacts to sculp- 
ture, normally received a high polish in the ancient Western world, and apparently in the 
East as well, and effort was made to maintain this polish. Those that were not maintained 
turned the familiar reddish-brown, and the old bituminous coating (if present) turned to a 
darkish color. Those statues exposed to damp outdoor or underground conditions later 
became tinged with greenish corrosion products. Above and below ground, and apparently 
beneath the ocean as well, the original high polish itself afforded a certain degree of 
corrosion resistance, for the smoother the metallic surface, the less well it can retain 
water, and the rubbing and burnishing process removes the softer constituents from the 
metallic surface to produce an exposed surface that is both enriched in the harder consti- 
tuents and, most important, has greater structural and compositional uniformity. Gettens 
has, for example, pointed out this phenomenon in explaining the thicker, reddish corrosion 
products found in the unpolished recesses of relief patterns on certain bronze Chinese 
vessels, compared with the smooth green formed on the polished areas of relief [36]. 

With the rebirth of bronze casting during the Renaissance a new element of value entered 
the picture, namely "antiqueness" of a bronze object. There is even some evidence that 
this concept was valued in Roman times by collectors of Greek bronzes. During the Renais- 
sance, however, it becomes clear that corrosion products had acquired a value, not only 
because occasionally the color effects of excavated antique bronzes could be quite alluring 
in themselves, but also because they testified to the age and therefore genuineness of an 
ancient bronze. And once this value was established, the imitators and forgers were not far 
behind. 

Coloristic effects were still primarily achieved through gilding or the use of con- 
trasting alloys on small objects, and by far the most popular and most common finish for 
bronzes large and small was the application of a dark lacquer, as can be seen in Donatello's 
David of aa. 1430, Which was the first free-standing, full-length bronze figure cast since 
Antiquity. The use of this dark lacquer may have been a misinterpretation of Pliny, or an 
imitation of an ancient, discolored, bituminous coating, or, more likely, such a coating 
served the important purpose of providing visual uniformity to conceal numerous casting 
flaws and repairs that are reportedly characteristic of Florentine 15th century bronzes. 
When, for example, the dark coating plus accretions and corrosion products were cleaned from 
the bronze doors executed by Fil arete between 1433-45 for St. Peter's basilica in Rome, they 
were found to be not only full of numerous original repairs but also made of a patch-work 
variey of alloy compositions clearly not intended to be seen [37]. In Donatello's earlier 
St. Louis of Toulouse of 1423, the statue was ingeniously cast in pieces small enough so 
that each could be gilt separately and the whole then put together much as a tailor constructs 
a suit [38]. The variety and experimental qualities of Donatello's approach to coloration 
of sculpture is exemplified as well by the wooden statue of the Magdalen which was covered 
for many years with a uniform brown paint to resemble bronze until the cleaning after the 
Florence flood revealed Donatello's bright polychromy [39]. 

The first modern account of the subject of the coloration of bronze occurs with the De 
Saulptura published in 1504 by Pomponius Gauricus, an amateur who made observations in the 
bronze foundries at Padua: "All beauty," he says, "appears perfect in the polishing and 
coloration. In the polishing we remove all harshness of the filing by means of a scraper, 
and we add the shine with pumice or with a point or with a burnisher. For coloration we 
give the color to each part whether in the cast itself (i.e.^ by alloying) or, he goes on 
to describe the following colors: "white is achieved by the application of silver leaf, 
yellow, i.e. J gold, with gold leaf; green by wetting with salted vinegar, and black by a 
varnish of liquid pitch or smoke of wet straw. These colors will do for now, in waiting for 
the time that we will learn others" [40]. 

Our other principal literary source for Renaissance practice comes from the writings of 
the painter Vasari whose Preface to his Vite or Lives of the Great Artists, published in 



83 



1550 and revised in 1568, describes various artistic techniques. Vasari simply states that 
bronze, "assumes through time and by natural change a color that draws toward black... Some 
turn it black with oil, and others with vinegar make it green, and others with varnish give 
it the color of black so that everyone makes it come as he likes best" [41]. 

Though Vasari mentions green formed by exposure to vinegar, artificial green patinas 
are rarely found until well into the 19th century, at least one reason for which is the lack 
of stability of artificially formed greens, particularly under outdoor conditions. 

The dark, shiny lacquer was most typical during the Renaissance, with certain exceptions, 
for example in the coloristic effects, particularly in small bronzes, achieved through 
combinations of metals and other materials seen in the work of such artists as Benvenuto 
Cellini, and Antico, and in the exquisite, translucent, reddish-brown lacquers typical of 
the work of Gian Bologna and his followers in the late 16th century [42]. 

As to stone and marble sculpture, it appears that in Roman times where carved represen- 
tations were largely derived from Greek originals from which the polychromy had disappeared, 
a dichotomy developed for the first time between high-class, unpolychromed sculpture and 
more popular and conservative works that were polychromed in the old tradition. This 
tendency was followed, by and large, through the Renaissance onward, and marble carvers 
adhered to the progressive classical tendency following the Roman traditions and never 
used color. For coloristic effects it was necessary to rely uopn textural variation. 

Rudolf Wittkower's discussion of the problem of the polychromy of sculpture, in an 
article about Bernini's bust of Louis XIV, points out the problem of the eye as having 
always presented sculptors with one of their greatest problems in representing a head, for, 
"of all parts of the human body, the eye alone has a design in it which exsists only in 
terms of color and not of shape— (that is) the iris and pupil. The problem is to translate 
this colored design into colorless sculptural form... Not until the Hellenistic period was 
a way found of representing the eye by purely sculptural means. Sculptors then depicted the 
iris by a circle cut out of the eyeball and the pupil either as one or two small holes in 
the center. While the shadow of the two holes gives the effect of the dark pupil, the ridge 
between them stands out clearly and produces an effect similar to the dash of light which 
enlivens the human eye. Since in real life this spot of light shifts with a person's angle 
of vision, the ridge enabled the sculptor to fix the direction of the look. The Romans 
accepted at certain periods the Hellenistic sculptured eye, while at others they preferred 
the simple Greek eyeball; but since they had abandoned polychromy, they left the eyeball 
unpainted... (During the Renaissance) one and the same sculptor would revert to the simple 
convex as well as to the sculptured eye... Michelangelo used the sculptured eye for his 
David, where he wanted the stare in the eyes to be fixed and determined. The same applies 
to his Moses. But in his Madonnas and statues in the Medici chapel he left the eyeball 
unworked" [43]. 

Few sculptors have expressed the general problem of polychromy in sculpture as well as 
has Gian Lorenzo Bernini in the 17th century, who was, to quote Wittkower, "always meditating 
upon the central question of portraiture in stone, namely how to translate the colors and 
the complexion of a face into uncolored marble," or we might add, into monochromatic bronze. 
Bernini stated that, "If a man whitens his hair, his beard, his lips, and his eyebrows and 
were it possible, his eyes, even those who see him daily would have difficulty recognizing 
him." He explained that, for instance, "in order to represent the bluish color which people 
have round their eyes, the place where it is to be seen has to be hollowed out, so as to 
achieve the effect of this color and to compensate in this way for the weakness of sculpture 
which can only give one color to matter. Adherence to the living model therefore is not 
identical to imitation" [44], presaging Picasso's famous statment that, "We all know that 
Art is not truth. Art is a lie that makes us realize truth..." [45]. 

A document referring to a bronze portrait bust by Bernini states specifically that he 
wished to reserve the final chasing of the face, hair and beard for himself, and that he 
preferred the bust without any type of coloring, "since time will give the metal a true and 
natural color" [46]. He was referring in this case to indoor bronze. In Bernini's immense 
bronze decorations for St. Peter's basilica in Rome, the Baldaaahino , the Cathedra Petri, 
and the Cappella del Saaramento , the bronzes were either all or partially gilt or highly 
polished and allowed to darken naturally to a rich red-brown. Coloristic effects were 
achieved by gilding and the contrasts with other richly colored materials such as marbles 
and lapis lazul i . 



84 



It is not until the 19th century and particularly in France that we find the develop- 
ment of what can be called true artificial patination practiced on a large scale. As late 
as 1802, Francesco Carradori's book on sculpture technique written in Florence describes 
only filing and polishing the bronze surface [47]. By mid-century, in Paris, it has been 
estimated that about 6,000 men were continuously employed in bronze casting [48]; among them 
were the Master Fatineurs, artistic specialists in their own right, who developed the art 
of coloring bronzes by application of heat and chemicals, and who, for the most part, care- 
fully guarded their secrets. It was the exception for the artist himself to apply his own 
patinas. The Limet brothers, shown here (fig. 5) in their Paris studio, did much of the 
patination of Rodin's bronzes. An exceptionally fine example of artificial patination is 
seen in this small horse by Degas (fig. 6) cast by the founder Hebrard after Degas' death in 
1917. Degas' use of polychromy in sculpture is exceptional but explainable by the fact that 
he was primarily a painter. Many of the small sculptures he left after his death were in 
multicolored waxes. The bronze caster Hebrard stated that particular efforts were made to 
duplicate the colors of the waxes in the coloration of the bronze. The horse is a particularly 
beautiful example of color variation by chemical means, with the horse colored a rich chestnut 
brown and the base tinged with green. Degas' interest in the coloration of form is perhaps 
best exemplified in the Little Dancer, Fourteen Years Old from 1880-81, exhibited in wax form 
during Degas' lifetime. The Dancer's shoes and bodice were real objects covered with colored 
wax and the sculpture was fitted with a real cloth ribbon and tutu. Such innovative effects 
were received with a certain amount of shock during Degas' lifetime [49]. 




Figure 5. Limet brothers, "Master Figure 6. Horse by Degas: a small indoor bronze 
Patineur," (Malvina Hoffman, with patina in good condition. 

Sculpture Inside and Out). 



While the Master Patineurs delighted in creating artistic effects on a small scale, 
large works were normally given the simplest of treatments, normally uniform and depending, 
it seems, largely on the quality of the cast. For example at the von Mtiller foundry in 
Munich, the bronzes were left uncolored and probably simply waxed. Harriet Hosmer's statue 
of Senator Thomas Benton, cast by von Mtiller in the 1860's was described as being a bright 
golden color at its unveiling [50]. Otherwise, large scale sculpture was normally chemically 
treated to turn a dark brown or black. 

85 



Efforts to study the change in appearance of outdoor bronzes wrought by atmospheric 
pollution at the dawn of the industrial age and to provide a more scientific base for the 
chemical coloration of bronze appears to have begun in Berlin in the 1860's. The various 
milestones can be found for the most part in the Lewin-Alexander "Patina Bibliography" [51]. 
Alongside this tradition is that of the sculptor's or craftsman's handbooks that present the 
various methods handed down in the workshops or, more often, simply lifted without acknow- 
ledgement from previously published handbooks. I have collected some eighteen or so books 
and articles by sculptors or metal-workers, the earliest dating from 1873, and there must be 
many more [52]. Malvina Hoffman's book, Sculpture inside and Out, first published in 1939, 
is among the most comprehensive accounts drawing on her experience as a student of Rodin and 
work with Master Patineurs in the Rudier foundry in Paris. For the most part these are 
simply recipe collections, with ingredients often described in obscure, arcane or imprecise 
fashion, for example: salt of sorrel, sulphydrate of ammonia, uric acid (for urine), wine 
vinegar, and the like. Explanations of the chemistry involved, when provided, are usually 
incorrect. It is no wonder that the various sculptor-authors often speak of the element of 
chance or luck in achieving desired results. Occasionally these handbooks provide a state- 
ment regarding desirable aesthetic qualities of artificial patination, all of which agree 
with that of Slobodkin who states that artificial patinas, 1) should appear natural, and 2) 
the patina should be very thin and transparent, and should emphasize the metallic qualities 
of the medium [53]. Uniformly absent is any discussion or even concern with maintenance 
other than the usual suggestion that bronzes should receive an occasional application of 
beeswax or commercial paste wax. One author, J. Rood, goes so far as to state: "If a 
sculpture is of sufficient importance that subsequent generations would like to preserve it 
indefinitely, a way can certainly be found" [54]. To this we should certainly exclaim, 
"Such faith!" 

Most modern sculptors in bronze believe in the romantic myth of a benevolent Nature 
that will in time provide their sculptures with a handsome patina. For example, Henry Moore 
has stated that, "...bronze, naturally in the open air (particularly near the sea) will turn 
with time and the action of the atmosphere to a beautiful green. But sometimes one can't 
wait for nature to have its go at the bronze, and you can speed it up by treating the bronze 
with different acids which will produce different effects. Some will turn the bronze black, 
others will turn it green, others will turn it red. I usually have an idea, as I make a 
plaster, whether I intend it to be a dark or a light bronze, and what colour it is going 
to be. When it comes back from the foundry I do the patination and this sometimes comes off 
happily, though sometimes you can't repeat what you have done other times... It is a very 
exciting but tricky and uncertain thing, this patination of bronze" [55]. 

Those of us who must be concerned with the preservation of bronzes and therefore with 
the problems of patina have a difficult task indeed, demanding the broadest use of scientif- 
ic, historical and aesthetic tools available. For sculpture the problem of patination is 
particularly subtle and acute, for when one considers the traditional way in which the 
sculptor has worked since the Renaissance in monochromatic wax or clay translated to plaster, 
translated to bronze or marble, his coloristic effects, as Bernini has stated so well, are 
dependent upon textural variation. For the eye to see these variations in texture and form 
requires a reasonable uniformity of coloration. By comparing the appearance of sculpture 
before and after conservation treatment, the camouflaging effect of a deteriorated patina is 
plainly apparent (figs. 7 and 8). 



86 




References 

[1] Cesare Brandi , "^he Cleaning of Pictures in Relation to Patina, Varnish and Glazes, 
Burlington Magazine , XCI, 556, July 1949, pp. 183-188. 

Neil Maclaren and Anthony Werner, Some Factual Observations about Varnishes and Glazes, 
Burlington Magazine, XCII, 568, July 1950, pp. 189-192. 

E. H, Gombrich, Dark Varnishes: Variations on a Theme by Pliny, Burlington Magazine, 
CIV, 707, February 1962, pp. 51-56. 

Otto Kurz, Varnishes, Tinted Varnishes and Patina, Burlington Magazine, CIV, 707, 
February 1962, pp. 56-60. 

S. Rees Jones, Science and the Art of Picture Cleaning, Burlington Magazine, CIV, 707, 
February 1962, pp. 60-62. 

J. PI esters, Dark Varnishes--Some Further Comments, Burlington Magazine, CIV, 716, 
November 1962, pp. 452-460. 

D. Mahon, Miscellanea for the Cleaning Controversy, Burlington Magazine, CIV, 716, 
November 1962, pp. 460-470. 

J. A. van de Graaf, The Interpretation of Old Painting Recipes, Burlington Magazine, 
CIV, 716, November 1962, pp. 471-475. 

M. Muraro, Notes on the Tradtitional Methods of Cleaning Pictures in Venice and Florence. 
Burlington Magazine, CIV, 716, November 1962, pp. 475-477. 



87 



[2] Brandi , op. ait. 
[3] Maclaren and Werner, op. ait. 
[4] Mahon, op. ait., p. 461. 
[5] Rees Jones, op. ait. , p. 60. 

[6] S. Lewin and S. Alexander, The Composition and Structure of Natural Patinas, Part I. 
Copper and Copper Alloys, Section A, Antiquity to 1929; Section B, 1930 to 1967, Art 
and Arohaeology Teahniaal Abstraats, VI, 4, 1967; and VII, 1, 1968. 

[7] G. Magnus, Uber die Einfluss der Bronzezusatnmensetzung auf die Erzeugung der schonen 
grunen Patina, Dinglers Polyteahnisah.es J. , 172, 1864, pp. 370-376. 

J. Riederer, Corrosion Damage on Bronze Sculptures, preprint of paper presented to ICOM 
Committee for Conservation, Madrid, October 1972, p. 4. 

J. Lehmann, Corrosion of Monuments and Antiquities made of Copper and Copper Alloy in 
Outdoor Exhibits, preprint of paper presented to ICOM Committee for Conservation, 
Madrid, October, 1972, pp. 3-4. 

[8] Webster's New World Dictionary, 2nd College Edition, D. Guralnik, ed, N.Y., 1970. 

[9] See, e.g., Webster's New World Dictionary, op. ait. 

[10] Filippo Baldinucci, Voaabolario Tosaano dell' Arte del Disegno , Florence, 1681. 

"Patena, voce usata da' Pittori, e diconla altrimenti pelle, ed e quell a universale 
scurita che il tempo fa apparire sopra le pitture, che anche talvolta le favorisce." 

[11] Battisti and Alessio, Dizionario Etimologiao Italiano , Florence, 1954. {patina) 

The Cambridge Italian Dictionary, vol. I, B. Reynolds, ed. , Cambridge, 1962. {patina) 

[12] Diderot and D'Alembert, Enayolopedie ou dictionnaire raisonne des sciences, des arts et 
des metiers, par une societe de gens de lettres , 36 vols., pis., Neufchastel , 1765, 
Lausanne, 1780-82. 

patine "II n'y a point de mot francois pour exprimer cette belle & brilliante couleur 
de vert-de-gris que le cuivre ne prend pas toujours; I'agrement de cette couleur pour 
I'oeil & la difficulty de la renconter (car tous les cuivres ne s'en chargent pas 
egalement), la rendent tres-recommandable aux Italiens, qui la nomment patina , comme 
on ose ici le faire d'aprfes eux, & par I'exemple de M. le comte de Caylus. 'II doit 
etre permis, dit-il avec raison, d' adopter un mot etranger au moins dans la langue 
des arts'. Or 1 'Encyclopedie en est le dictionnaire." 

[13] Battisti and Alessio, Dizionxwio Etimologiao Italiano, op. ait. 

[14] See, e.g., C. S. Smith, A History of Metallography, 2nd ed. , Chicago, 1965, p. 2; J. 
R. Partington, Origins and Development of Applied Chemistry, London, 1935; A. J. 
Hopkins, Alchemy, Child of Greek Philosophy, N.Y. 1934. 

[15] Erich Pernice, Untersuchungen zur antiken Toreutik, V. Natiirliche und kiinstliche 

Patina im Altertum, Jahreshefte des Osterreiahisahen Arahdologischen Institutes in 
wien, XIII, 1910, pp. 102-107; Erich Pernice, Bronze Patina und Bronzetechnik im 
Altertum, Zeitsakrift fur Bildende Kunst, XXI, 1910, pp. 219-224. 

[16] G. M. A. Richter, The Metropolitan 14useim of Art, Greek, Etruscan, and Roman Bronzes, 
New York, 1915. 



88 



[17] H. Otto, X-ray fine structure investigation of patina samples, Freiherger Forsahungshefte , 
B 37, 1959, pp. 66-77. For further discussion of ancient practices see, e.g. : 
Charbonneaux, Greek Bronzes, London, 1962, pp. 36 ff. who mentions that as early as 
1896, Villenoisy wrote in the Revue Arahaiologique against the supporters of artificial 
patination, pointing out how many Greek and Roman kitchen utensils with the humblest 
functions have a splendid patina. 



[18] G. M. A. Richter, op. ait., pp. xxix-xxx. 



[19] Vitruvius, On Arohiteoture, 2 vols., tr. ed. Frank Granger, Loeb Library, Cambridge, 
Mass., 1970, vol. II, p. 119 (VII. ix. 3). 

P. Reutersward, Studien zur Polyahromie der Plastik Grieohenland und Rom, Stockholm, 
1960. 



M. Cagiano de Azevedo, Conservazione e restauro presso i Greci e i Romani, Bollettino 
dell'Istituto Centrale del Restauro, 9-10, 1952, pp. 53-60. 

[20] Pliny, natural History, 10 vols., Loeb Library, vol. IX, tr. H. Rackham, London, 1968 
(XXXIV. xxi). 



Pernice, Osterreiohisohe Jahreshefte, op. ait. 
G. M. A. Richter, op. ait., p. xxx. 



[21] Richter, loa. ait. 



[22] Pernice, Osterreiahisahe Jahreshefte, op. ait., quoted in Richter, op. ait., p. xxxi . 



[23] Pliny, op. ait. XXXIV. 
[24] Pliny, op. ait., XXXIV. xxvi. 
[25] Pliny, op. ait., XXXIV. xx. 
[26] Pliny, op. ait., XXXIV. iii. 
[27] Pliny, op. ait., XXXIV. x1. 

[28] Pliny, op. ait., XXXIV. xxv; used also for the same purpose by Benvenuto Cellini, 

Trattato dell' Orefioeria e della Saultura, 1568, ed. Milanesi, 1857, tr. C. R. Ashbee, 
The Treatises of Benvenuto Cellini on Goldsmithing and Sculpture, N.Y. 1957; and by 
Pomponius Gauricus, Be Saulptura, 1504, ed. Andre Chastel and Robert Klein, Geneva, 
1969, p. 232, and pp. 232-233, n. 47. 

[29] Pliny, op. ait., XXXIV. xxi. 

[30] Pliny, op. ait., XXXIV. ix. The passage reads: "bitumine antiqui tinguebant eas, quo 
magis mirum est placuisse auro integere." At least part of the problem is the trans- 
lation of tinguehant, which the Loeb Library edition, for example, translates as used 
to stain. In this case, it apparently means aoated, as Pernice convincingly demonstrates 
(see above, n. 15) . 



[31] Richter, op. ait., p. xxx. 

[32] Pliny, Natural History, XXXIV. ix. 

[33] E. H. Gombrich, op. ait. ; and 0. Kurz, op. ait. 

[34] Mahon, op. ait. p. 461. 



89 



[35] Vittorio Federici, in Le Povte Bizantine di San Motqo, a cura della Procuratoria di S. 
Marco, edizioni dello Stadium Cattolico Veneziano, Venezia, 25 April 1969. The inscrip- 
tion reads: "Rogo et adiuro vos, rectores S. Angel i Michael is, ut semel in anno 
detergere faciatis has portas, sicut nos ostendere fecimus ut sint semper lucidae et 
clarae." 

[36] R. J. Gettens, The Freev Chinese Bronze a , vol. II, Teohniaal Studies, Freer Gallery of 
Art, Oriental Studies, no. 7, Washington, 1969, p. 182. 

[37] Verbal communication from John Spencer. 

[38] Bruno Bearzi, Considerazioni di tecnica sul S. Ludovico e la Giuditta oi Donatello, 
Bollettino d'Arte, xxxvi , 1951, pp. 119-123. 

[39] See e.g., the photograph of the Magdalen in Dora Jane Hamblin, Science finds way to 
restore the art damage in Florence, Smithsonian, IV, 11, February 1974, p. 29. 

[40] Ponponius Gauricus, op. ait., p. 230. 

[41] Giorgio Vasari, Le Vite..., 1550, 1568, G. Milanesi, ed., Florence 1878; "Prefix" to 
the Vite tr. and ed. Maclehose and Brown, Vasavi on Technique, N.Y., 1960, pp. 165-166. 

R. Bettarini and P. Barocchi , eds., Verona, 1966, vol. I, p. 103. 

"Questo bronzo piglia col tempo per se medesimo un colore che trae in nero e non in 
rosso come quando si lavora. Alcuni con olio lo fanno venire nero, altri con I'aceto 
lo fanno verde, et altri con la vernice li danno il colore di nero, tale che ognuno 

10 conduce come piu gli piace. Nel che si vede questa arte essere in maggior eccellenza 
che non era al tempo degli antichi." 

[42] Further on Renaissance bronzes, see: W. Bode, with M. Marks, The Italian Bronze Statuettes 
of the Renaissance, 3 vols. London, 1908-12. 

H. Liier, Teahnik der Bronzeplastik , Monographien des Kunstgewerbes , IV, ed. J. L. 
Sponsel, Leipzig, n.d., p. 16 ff. on patina. 

J. Montagu, Bronzes, London, 1963. 

H. R. Weihrauch, Europaisohe Bronzestatuetten, Braunschweig, 1967. 

G. Mariacher, Venetian Bronzes from the Collections of the Correr Museum, Venice, 
catalogue, 1965-69, bibliography. 

[43] Rudolf Wittkower, Bernini, The Bust of Louis XIV, London, 1951, pp. 9-11. 

[44] Ibid. p. 9. 

[45] A. H. Barr, Jr., Picasso, Fifty Years of his Art, N.Y., 1946; reprint, 1966, p. 270. 

[46] Gisela Rubsamen, Bernini and the Orsini Portrait Busts, lecture. College Art Association, 
22-25 Jan 1975, Washington, D.C., Abstracts. The only other source for patination in 
the 17th century that I know of is Andre Felibien, Des Principes de I ' Architecture , de 
la Sculpture, de la Peinture, et des autres Arts qui en dependent, Paris, 1699, Farn- 
borough, Hants., England, 1966, p. 239: 

"Apres qu'elles sont bien nettoyees & reparees, on leur donne si I'on veut une couleur. 

11 y en a qui prennent pour cela de THuile & de la Sanguine: d'autres les font 
devenir vertes avec du Vinaigre. Mais avec le temps la bronze prend un vernis qui tire 
sur le noir." 

The French Enayalopedie of Diderot (op cit.) states (under sculpture): "Quant a la 
poix dont les anciens couvroient leurs bronzes, nous n'avons rien a desirer; les 
fumees & les preparations de nos artistes sont d'autant preferables, qu'elles ont moins 
d' epaisseur." 



90 



[47] Francesco Carradori, Istruzione Elementare per gli studioa-i della Scultura, Florence, 
1802, p. xxxiii. 

"Finalmente ripurgasi il lavoro, e conducesi alia dovuta perfezione, prima con lime 
diverse, piu o meno fini, e capaci di entrare in tutte le parti; indi con acqua e 
pomici di varie grossezze; e per ultimo con dell'istessa pomice pesta, e stecche di 
legno, e con tripolo." 

[48] G. Savage, A Concise History of Bronzes, N.Y., 1968, p. 227. 

[49] On Degas sculpture see: John Rewald, ed.. Degas, Works in Sculpture, a Complete 
Catalogue, N.Y. 1944. 

[50] C. Carr, Harriet Hosmer, Letters and Memoirs, N.Y., 1912, p. 260. 

[51] S. Lewin and S. Alexander, op. cit., see, e.g.: 

1864/Magnus, op. ait. 

1883/C. Puscher, Artificial patinas, Polyteahnieahes Notizblatt, 38, 90. 

1884/E. Donath, Artificial patination, Dinglers Polytechnisohes Journal, 253, 376-80. 

1903/L. Vanino and E. Seitter, Patina. Its Natural and Artificial Formation on 
Copper and its Alloys, Vienna. 

1911 /A. H. Hiorns, Metal-Colouring and Bronzing, London, 2nd ed. 

1925/S. Field and S. R. Bonney, The Chemical Coloring of Metals and Allied Processes, 
N.Y. (especially Chapter XI, pp. 137-152). 

1925/C. Fink and C. H. Eldridge, The Restoration of Ancient Bronzes and other Alloys, 
N.Y. 

1932/J. R. Freeman and P. H. Kirby, The Rapid Development of Patina on Copper, Metals 
and Alloys, III, pp. 190-194. 

W. H. J. Vernon, Open-air corrosion. .. Ill— Artificial production of a green 
patina, J. Inst. Metale , 49, pp. 153-61. 

1962/D. Fishlock, Metal Colouring, Teddington, (chapter 11, pp. 192 ff.). 

1 974/DeutSChes Kupfer-InstitUt, Chemisahe Fdrhungen von Kupfer und Kupferlegierungen 
[DKI, 1 Berlin 12 (Charlottenburg) , Knesebeckstrasse 96, Germany]. 

[52] 1873/Ernest Spon, Workshop Receipts, for the use of manufacturers, mechanics, and 
scientific amateurs, London, 1873; reissued 1875 and 1890; 2nd ed. 1895. 

1891/Georg Joh. A. Buchner, Die Metallfdrbung und deren Ausfuhrung, mit hesonderer 
Berucksiohtigung der chemischen Metallfdrbung , Berlin, 1891; revised ed. 1920. 

1895/W. 0. Partridge, The Technique of Sculpture, Boston, 1895. (Patina on Bronze, 
pp. 90 ff.). 

1912/H. Maryon, Metalwork and Enamelling, London, 1912; 5th revised ed., N.Y. 1971. 
("The colouring of copper and brass," pp. 264-266). 

1938/Hugo Krause, Metal Coloring and Finishing . New York, 1938. 

1939/Malvina Hoffman, Sculpture inside and Out, New York, 1939. ("Bronze Patining," 
pp. 302-304). 

1940/Ralph Mayer, The Artist's Handbook of Materials and Techniques, N.Y., 1940; 3rd 
ed. 1970. ("Patina on Bronze," pp. 618-620). 

91 



1947/Jack C. Rich, Mateviale and Methods of Sculpture, N.Y. 1947, 1970. ("The Patination 
of Metals," pp. 199-209.) 

William Zorach, Zoraah Explains Sculpture, N.Y., 1947. (patination, pp. 161 ff.) 

1949/Louis Slobodkin, Sculpture, Principles and Practice, Cleveland, 1949. ("Patining 
Bronze," pp. 166 ff.) 

1952/Jules Struppek, The Creation of Sculpture, N.Y., 1952. (patinas, pp. 232 ff.) 

1960/Bernard Chaet, Artists at Work, Cambridge, Mass., 1960. 

1963/John Rood, Sculpture with a Torch, Minneapolis, 1963. (patinas, pp. 36 ff.) 

1965/John Mills, The Technique of Sculpture , London, 1965; 3rd ed. 1967. (ch. 14, 
"Finishes," pp. 106 ff.) 

1965/John Brzostoski , Patination of Bronzes, Craft Horizons, Nov. -Dec. 1965, pp. 
26-28. 

1966/D. Meilach and D. Seiden, Direct Metal Sculpture, N.Y., 1966. ("Color and Patina," 
pp. 35 ff.) 

1967/John Baldwin, Contemporary Sculpture Techniques, N.Y., 1967. (patinas, pp. 60 ff.) 
1968/Nathan Cabot Hale, Welded Sculpture , N.Y., 1968. (patinas, pp. 126 ff.) 

1970/Donald J. Irving, Sculpture, Materials and Process, N.Y., 1970. (ch. 9, "Finishing," 
pp. 127 ff; patination, pp. 136 ff.) 

1972/Jean de Marco, Bronzes and their Patinas, National Sculpture Review, I, vol. XXI, 
no. 1, Spring 1972, pp. 23-25; II, vol. XXI, no. 2, Summer 1972, pp. 25-26. 

[53] L. Slobodkin, op. cit. , p. 166. 

[54] J. Rood, op. cit., p. 39. 

[55] Henry Moore, Henry Moore on Sculpture, Philip James, ed., N.Y., 1967, p. 140. 



Discussion 

J. Olin: Does the corrosion on outdoor bronzes serve as a form of protection against 
future corrosion? 

P. Weil: We have examined and photographed the surfaces of a number of outdoor bronzes 
60-100 years old and have found them to be very badly pitted. The pitts average up to 2 mm 
in depth. While a certain degree of protection is provided by the basic copper sulfate, it 
appears that it is not total and in fact corrosion is still going on, much more actively in 
some places than in others, particularly in green areas. 

T. D. Weisser: I am not sure that the green product should be removed; however, after 
working on several outdoor bronzes, I would agree with Phoebe Weil, that the green 
corrosion product on these bronzes is not necessarily protective. On the bronzes I have 
worked with, the areas with green corrosion are lower than the areas under the black 
spots, which could indicate that the green areas have corroded more heavily than those 
covered with the black product. Often there is a difference of 1/16 inch in the levels 
of the metal under these two areas. 

K. E. Holm: You might be interested in knowing that we in our museum have examples of 
bronze spearheads colored with what is supposed to be a bitumen. The bitumen docoration 
is well preserved and stands black in contrast to the now somewhat green bronze. The 
spearheads are dated to about the 3rd-5th century A.D. 



92 



THE PRODUCTION OF ARTIFICIAL PATINATION ON COPPER 



D. C. Hemming 

Revere Copper and Brass, Inc. 
Rome, New York 



Patination on copper is a natural process that has been observed for centuries on 
copper roofs and on copper-base metal artifacts that have been exposed to the action of 
the atmosphere. The colors developed during natural patination are generally a pleasing 
green, though each local area often will have a particular shade of green that is its own 
characteristic. The shade of green may be modified by impurities in the copper as well as 
by the local form of air pollution (fig. 1). 



Amsterdam, 
NETHERLANDS 
1830 



I 



1 




Munich, 


1 Vienna 1 


GERMANY 


AUSTRIA 


1934 


1880 












Copenhagen, 
DENMARK 
1714 



Copenhagen, 
DENMARK 
1939 



Stocl(holm, 
SWEDEN 
1650 



Brussels, 
BELGIUM 
1932 



Santiago, 
CHILE 
1937 



r 





Sydney, 
AUSTRALIA 
1830 



Oxford, 
ENGLAND 
1753 



Devonshire, 
ENGLAND 
1906 




Figure 1 . 
Natural 
various 



Range of possible shades of patinas on copper (see color plate r). 
patinas formed on copper after extended atmospheric exposure in 
parts of the world. International Copper Research Association, Inc. 

93 



I 



This figure was furnished by the International Copper and Brass Research Association 
and illustrates the range of possible shades of green. The specimens were assembled from 
worldwide locations. In the top row from the left, we have represented Amsterdam installed 
in 1830, Munich in 1934, Vienna in 1880, and Philadelphia in 1737. The second row includes 
Copenhagen in 1714, Copenhagen in 1880, Paris in 1944, and Buston, England, undated. The 
third row consists of another from Copenhagen in 1939, Stockholm in 1650, Brussels in 
1932, and Santiago in 1937. The bottom row has specimens from Sydney in 1830, Oxford in 
1753, Devonshire in 1906, and Berlin in 1800. 

It was thought, for many years, that natural patination consisted of basic copper 
carbonate either in the form of azurites or malachite. This misconception has persisted 
for a long time even though as early as 1908, it was reported by Loock that the patina on 
the Jan Wellem Memorial in Dusseldorf was basic copper sulfate rather than basic copper 
carbonate. From 1923 to 1933, W. H. J. Vernon and collaborators investigated natural 
patinas extensively and came to the same conclusion that the major constituent of most 
natural patinas was brochantite or basic copper sulfate. Vernon and Whitby sampled patinas 
from urban, rural, marine, and urban marine locations in England. Careful chemical analysis 
established that copper carbonate was only a minor constituent and that the only exception 
to the predominance of brochantite occurred in a marine environment that was free from 
atomospheric pollution. Natural patinas formed in a clean marine environment were found 
to consist of basic copper chloride. 

As recently as 1962, F. L. LaQue and W. D. Mogerman referred to the patina on the 
Statue of Liberty as being a "handsome protective layer of green, basic copper sulfate." 
This statement aroused opposition from several quarters and was finally resolved by chemical 
and x-ray diffraction analysis performed by D. H. Osborn of American Brass who confirmed 
that the patina was composed predominantly of basic copper sulfate with less than 5 percent 
basic copper chloride and less than 0.1 percent basic carbonate. 

There have been many methods developed to produce a green coloration or patina on 
copper or copper base artifacts by accelerated means. The natural process takes approxi- 
mately ten years and for most of that time the surface has a dark brown to black appearance 
that is not particularly pleasing. Copper will form green or blue colored salts with most 
of the common acids, therefore, it is not surprising that the reagents used in the solutions 
devised for artificial patination seem to represent a large proportion of the stock found 
on many laboratory shelves. Some of the formulations that have been used are: 



1. Ammonium chloride 10 parts 
Sodium chloride 20 parts 
Copper carbonate 30 parts 
Cream of tartar 10 parts 
Acetic acid 1 :1 100 parts 

2. Ammonium chloride 8 parts 
Sodium chloride 8 parts 
Ammonium hydroxide 15 parts 
Acetic acid 500 parts 

3. Sodium chloride 20 parts 
Copper acetate 20 parts 
Ammonium carbonate 60 parts 
Cream of tartar 20 parts 
Acetic acid 100 parts 



4. 


Ammonium chloride 


10 parts 




Sodium chloride 


40 parts 




Copper nitrate 


80 parts 




Cream of tartar 


10 parts 




Acetic acid 


1000 parts 


5. 


Sodium chloride 


20 parts 




Copper nitrate 


20 parts 




Water 


100 parts 


6. 


Ammonium chloride 


250 parts 




Ammonium carbonate 


250 parts 




Water 


1000 parts 


7. 


Copper nitrate 


25 parts 




Zinc chloride 


25 parts 




Water 


100 parts 



It is recommended that all the preceding solutions be brushed on thinly to the grease- 
free copper surface and allowed to air dry. After drying, the surface is rubbed with a 
soft cloth. This process must be repeated several times in order to develop the depth of 
color desired. The skill and artistry of the person doing the work play a large part in 
its quality. 

Freeman and Kirby have proposed an alternate immersion process that used a 10 percent 
solution of ammonium sulfate at 140 °F. This solution must be conditioned or saturated 



94 



with copper at a pH of 5,5 to 5.7. The process is said to require at least 24 hours and a 
final dip in hot water to develop a natural color in the basic copper sulfate coating. 

The Incra Patine I Process is the spray application of an ammonium sulfate, copper 
sulfate, lithium chloride, sodium dichromate, and hydrochloric acid solution that is 
thickened with a silicate. This process can be adversely affected by the occurrence of 
inclement weather too soon after application. 

Svenska Metall verken, Sweden, has developed a prepatination process under the direction 
of Dr. E. Mattsson. In this process, the reaction product from cupric nitrate, ferric 
sulfate, and sodium hydroxide is applied to the preoxidized copper sheet. The coated 
sheet is dried under carefully controlled conditions and coated with a sealer. The coating 
resembles natural patina and is designed to last until the natural patina develops. It is 
claimed that adherence improves with exposure time and that the presence of sulfate can be 
detected in the coating after five years exposure. 

The preceding methods are and have been used more or less successfully, but none of 
them are as satisfactory and simple as the one bath dipping processes that are available 
for coloring copper and its alloys brown and black. In 1966, Professor C. K. Hanson of 
the University of Utah began a research program for Incra, directed towards the preparation 
of inorganic coatings on copper. The early months were devoted to finding combinations of 
reagents that would produce colored adherent coatings on copper. This was a trial and 
error procedure and employed elevated temperatures and pressures. 

It was found that a number of reagents would produce colored coatings provided an 
oxidizing environment at the proper pH was maintained. Recognizing that a pressurized 
system would be impractical, efforts were directed towards developing an effective cheap 
aqueous oxidizing system. It was found that potassium chlorate was the most efficient of 
the reagents that were tested. The coating produced on copper by potassium chlorate was 
determined to be cuprous oxide. When copper sulfate was added to the system a green layer 
formed that was identified as brochantite, the basic copper sulfate found in most naturally 
patinated copper. Cleaned copper panels immersed in dilute solutions of potassium chlorate 
and copper sulfate developed a coating of brochantite in about 20 days at room temperature. 
Further work indicated that the time required for brochantite formation could be reduced 
by agitation, increased solution concentration, and elevated temperature. 

As a result of this work. Revere Copper and Brass began work for Incra on a program 
designed to scale up this method of patination from the test tube/breaker size to a pilot 
operation in which several 3 foot x 4 foot sheets of commercial 16 ounce ETP copper (CDAllO) 
could be patinated at one time. It was proposed to investigate the effects of variations 
in agitation, temperature, and solution concentration on the rate of brochantite formation. 
Determination of solution stability, and the feasibility or need for rejuvenation of used 
solutions would be made by conducting frequent analyses for copper, sulfate, chloride, and 
chlorate ion concentrations. Redox and pH would be monitored during each run. The patinas 
formed would be evaluated with regard to color, uniformity, and adherence. Atmospheric 
exposure tests would be started using representative panels from each run. 

Initial experiments conducted on a beaker scale confirmed that the Utah Process could 
be reproduced in Revere' s Laboratory. A solution concentration of 2.5 g/1 each of copper 
sulfate pentahydrate and potassium chlorate was adusted to pH 3.0 with sulfuric acid. 
Three cleaning methods were investigated, sulfuric acid pickling, ammonium persulfate 
etching, and mechanical cleaning with abrasive-impregnated nylon pads. Mechanical cleaning 
produced the best results in a 20-day still exposure. Agitation by stirring reduced the 
time required for patination confirming observations made previously at the University of 
Utah. 

The pilot equipment consisted of a stainless steel tank, 60 inches long, 24 inches 
wide, and 49 inches deep. Its working capacity was 1060 liters. An overflow weir at one 
end fed a stainless steel centrifugal pump with a pumping capacity of 40 gallons per 
minute at 35 feet of head. The output of the pump flowed through a heat exchanger equipped 
with a thermostatically controlled immersion heater. After passing through a flow meter, 
the solution was distributed from a header to four perforated pipes located at the bottom 
of the tank. All materials in contact with the solution were Type 316 stainless steel. 
The maximum flow rate proved to be 17 gallons per minute, somewhat less than anticipated. 



95 



When charging the tank, a mixer was installed on the end of the tank. Deionized 
water was added to the working level and the pump and stirrer started. Additions of 
reagent grade copper sulfate and potassium chlorate based on the 1060 liter volume of the 
system were made and stirring was continued until the salts were dissolved. pH adjustment 
was made by adding 10 percent sulfuric acid while monitoring with a pH meter. The auxiliary 
stirrer was then removed and agitation continued by pump. 

Four holes were punched on the long side of the 3 foot x 4 foot x 16 ounce copper 
sheets and they were cleaned mechanically on both sides with abrasive impregnated nylon 
pads. The sheets were rinsed and suspended in the tank by stainless steel hooks on 
insulated cross bars. Twelve 3 inch x 3 inch cleaned copper panels were taped to one of 
the sheets for daily removal to check on the progress of patination. A solution sample 
was removed daily and checked for pH, redox potential, copper, chloride and potassium 
chlorate. 

A total of nine runs were made in this pilot equipment. The initial runs established 
that mechanial cleaning of large areas by hand was not satisfactory. The patination was 
soft and nonadherent and the nonuniformity appeared to be associated with cleaning. The 
agitation provided by the pumping system was also contributing to the nonuniformity (fig. 2a). 
This result is typical of the mirror image patterns developed on adjacent sheets by turbulent 
flow. By increasing the copper sulfate concentration and raising the temperature, we were 
able to develop a fairly heavy patina, but it was soft and nonadherent (fig. 2b). During 
these runs, the solution became turbid at about the same time that patination began to develop. 
Analyses of the soft nonadherent patina indicated that chloride ion was being incorporated in 
the patina. A run made without agitation at ambient temperature yielded a dense adherent 
patina with no chloride present. 




b 



Figure 2. Patinas produced in pilot equipment: 

a. typical non-uniform mirror image patterns developed on adjacent 
sheets when turbulent flow is present (see color plate b); 

b. heavy but soft and non-adherent patina produced when copper 
sulfate concentration and temperature were increased. 



The analyses and measurements made on the daily samples showed that the copper 
concentration increased rapidly during the oxidation stage and as patination proceeded, 
decreased to nearly the initial value. The redox potential fell rapidly during oxidation 
from 600 mV to 300 mV and then rose to 350-400 mV during patination. The pH increased 
rapidly during oxidation from 3.0 to slightly higher than 5.0, then leveled off at 5.0 
during patination. 



96 



As the pilot runs were in progress, considerable time was devoted to bench work in an 
effort to solve the coverage problems associated with agitation and surface preparation. 
Variations in the mode of exposure to the patination solution were also investigated. In 
one approach a cleaned 3 inch x 5 inch copper panel was supported at a 10° angle in a PVC 
box. The patinating solution was pumped from a heated battery jar over the panel and 
returned to the battery jar at velocities ranging from 50 to 1000 in/mi n and at temperatures 
from ambient to 130 °F. Results were negative, oxidation started normally, but then 
changed from a brown to yellow colored film. 

At the conclusion of this test it was observed that brochantite crystallization had 
developed on a submerged PVC surface that had been in contact with the heated bottom 
surface of the battery jar. A 3 inch x 3 inch cleaned copper panel was placed flat in the 
bottom of the battery jar, covered with a liter of fresh patinating solution and held at 
130 °F. A dense adherent green patina developed on the upper surface of the copper panel 
in 64 hours. A shallow pyrex tray large enough to hold a 5 inch x 8 inch panel and equipped 
with a cover was then used in place of the battery jar. Using a solution of 25 g/1 copper 
sulfate and 2.5 g/1 potassium chlorate at 180 °F, a dense adherent patina was developed in 
6 to 8 hours. This experiment was repeated using six 2 inch x 2 inch panels. A panel was 
removed each hour for 6 hours. The following series of figures are from a scanning electron 
microscope study of these panels that was made at the Pulp and Paper Research Institute of 
Canada. The magnification on the figure is XIOOO. Figure 3a is a copper surface as 
prepared for patination. In figure 3b, a panel was exposed for 1 hour in the patinating 
solution-cuprous oxide forming. At 2 hours (fig. 3c), the cuprous oxide formation is 
nearly complete. In figure 3d, brochantite formation has started at 3 hours. It is 
visible as clumps of monoclinic crystals. At 4 hours (fig. 3e) brochantite growth continues. 
As the development of brochantite continues (fig. 3f ) , the cuprous oxide is absorbed. At 
6 hours (fig. 3g), the growth of brochantite is complete with very little cuprous oxide 
visible. 

The acceleration in patination gained through the horizontal exposure is believed to 
be due to the heat flux through the panel generated by contact of the panel with the 
heated bottom of the tray. Panels suspended in a horizontal position, but not in contact 
with the heated bottom surface of the tray did not patinate any more rapidly than the 
vertical panels. This approach was not pursued further as its application on a larger 
scale did not appear feasible. 

The bench work on vertical exposure led to the installation of an immersion heater in 
the bottom of the pilot tank. This arrangement provided gentle thermal agitation while 
heating the solution. Dense, adherent patinas were produced in four days. The solution 
concentration was 25 g/1 copper sulfate and 2.5 g/1 potassium chlorate and was operated at 
150 °F. Figure 4a is representative of the patina developed. Figure 4b shows a natural 
patina said to be 50 years old. 

As a follow-up to the Incra Patine II program. Revere installed a small scale commercial 
operation. This was intended to establish operating procedures and costs while processing 
full size copper sheets (36 inch x 96 inch x 16 ounces). It was expected that experience 
gained in operating this line would determine whether a full scale operation would be 
feasible technically and economically. Installation of the prepatinated sheets by commercial 
roofers would indicate the response of the product to standard roofing practices. 

The small scale prepatinating line was designed to process 36 copper sheets, 
3 foot X 8 foot X 16 ounces, racked back-to-back per load. The spacing between racks was 
2 inches. The design production was to be one load per week. This allowed one day for 
unloading, loading, precleaning, and solution adjustment. The patination cycle was four 
days. 

Three processing tanks 4 foot x 4 foot x 10 foot (1 .2 m x 1 .2 m x 3 m) were constructed. 
Two tanks were mild steel and one was Type 316 stainless steel. The three tanks were 
installed adjacent to each other with the long dimension of the tanks at right angles to 
an overhead hoist line. A one ton hoist and accessory facilities for racking and drying 
serviced the line. A work handling carrier was loaded from the top and held 18 racks on 2 
inch centers. Two sheets were clamped together using appropriate lengths of slit tubing 
along each of the four sides. Stainless steel bolts and nuts provided the clamping pressure 
required to hold the double sheets firmly. 

97 





Figure 3. Scanning electron microscope study 
of patination on copper panels (X 1000). 

a. copper surface as prepared for patination; 

b. one hour in patinating solution, CU2O 
starting to form; 

c. two hours, CU2O formation nearly complete; 

d. three hours, brochantite formation start- 
ing; 

e. four hours, brochantite growth continues; 

f. as brochantite continues to form, CU2O 
is absorbed; 

g. six hours, brochantite growth is complete. 



98 



Figure 4a. Four day patina (scanning 
electron microscope, X 1000). 



Figure 4b. Fifty year old natural 
patina (scanning electron micro- 
scope, X 1000). 



The first tank in line was heated and used to hold the cleaning solution. The second 
tank was equipped with an overflow weir to the drain. This tank served as the rinse after 
cleaning and after patination. The third tank was Type 316 stainless steel and was equipped 
with a steam coil and temperature control. Cold water was provided at each tank and each 
tank had a bottom drain and valve. 

The cleaner was Northwest Chemicals AC-5 used 10 percent by volume at 165 to 170 °F 
(74 to 77 °C) and a pH of 5.0 to 6.0. This is a mildly acid detergent solution that was 
very effective in removing residual rolling lubricant and any light tarnish that may have 
been present. The patinating solution concentration used was 20 g/1 copper sulfate penta- 
hydrate, and 2.0 g/1 potassium chlorate at an initial pH of 3.0. The copper sulfate was a 
commercial grade as supplied by Phelps-Dodge Refining. The potassium chlorate was a 
technical grade, supplied by Sargent-Welch Scientific. Battery acid grade sulfuric acid 
was used for pH adjustment. Rome tap water, which is relatively soft was used for make-up 
and to replace evaporation losses. 

A typical operating cycle included the following steps: 

1. The copper sheets were racked and loaded into the carrier. 

2. The loaded carrier was immersed in the cleaner for 30 minutes. 

3. The carrier was rinsed for 60 minutes. 

4. The carrier was immersed in the patinating tank. The tank was covered and the 
steam turned on. 

5. The temperature of the patinating tank was checked daily and water added to make 
up for evaporation losses. 

6. At four days immersion time, the carrier was removed, rinsed for 30 minutes, and 
transferred to the load-unload area. 

7. When dry the patina ted sheets were unracked and packed in a shipping box. 

8. The patinating solution was analyzed for potassium chlorate and chloride. 
Additions of potassium chlorate were made to bring the total of KCIO3 + CI to 
2.0 g/1 and the pH was adjusted to 3.0 with sulfuric acid. 



99 



This cycle took a week under ideal conditions and was repeated on a weekly basis. 
Figure 6 is a view of the loaded carrier in the rinse tank. Figure 5 is a finished sheet 
on the way to the unracking area. Figure 7 is the finished work being replaced in the 
shipping box. 

Of course, ideal conditions did not prevail all the time; in fact, at times conditions 
were just the opposite. We soon found that our design spacing of two inches was too close 




Figure 6. Finished sheet on way to unracking area. 



100 



Figure 7. Finished sheet being replaced in a shipping box. 



and that to get good coverage the sheets had to be spaced four inches apart, thus cutting 
our production by half. 

In the large tank, the chloride ion produced during the oxidation of the copper by 
the potassium chlorate increased regularly with each load of sheets that was patinated. 
When the chlorides reached 0.6 to 7.0 g/1, the patination became dusty and nonadherent and 
the solution had to be discarded. A modified solution was developed using hydrogen peroxide 
as the oxidizer, but while this worked well on the laboratory bench it was unsuccessful in 
the production tank. 

We investigated several methods for chloride removal and settled on a procedure in 
which the copper was precipitated as basic copper carbonate using soda ash. After settling, 
the clear supernatant was discarded. The precipitated copper was redissolved with dilute 
sulfuric acid, potassium chloride was added at 2.0 g/1 and patination resumed. The chloride 
removal procedure was repeated three times on one solution when the chloride concentration 
reached 0.6 g/1. The time for patination became longer after chloride removal, so we 
abandoned the process except for the use of soda ash to recover the copper when it was 
necessary to discard a solution. A new solution would process six loads before it was 
necessary to change it. 

Another troublesome characteristic of the solution was the generation of loose or 
tramp brochantite crystals which accumulated in the bottom of the tank. The stainless 
steel rack members developed an adherent brochantite layer after a time. This adventitious 
brochantite made it difficult to adjust the pH to 3.0 as it reacted slowly with the sulfuric 
acid to raise the pH. This could be partially counteracted by periodically dissolving the 
brochantite off the rack members and by removing the accumulated loose brochantite during 
a solution change. 

We experienced a leakage problem in the stainless steel patination tank. This occurred 
during the early usage of the equipment at a time when the chlorides reached 1.0 g/1 and 
the tank was idle for a time. The leak was caused by pitting attack. After repairs, we 
had no other major problems with corrosion, though I feel that the process could have been 
improved by eliminating all dissimilar metals from contact with the solution and the 
copper. 

A total of 630 3 foot x 8 foot x 16 ounce sheets were processed. Thirty-six of these 
sheets were installed by a commercial roofer on a porch roof in Ontario, Canada. Another 
use was the 375 sheets installed on the Borough House, Sumter, North Carolina that is 
shown in figure 8. 



101 



Figure 8. Roof on Borough House, Sumter, North Carolina, constructed 
from artificial patina sheets (see color plate a). 

The color of the patina formed by this process is a dark green and does not resemble 
a natural patina initially. Our exposure tests which have been in progress for four years 
show that the Incra Patine II weathers very well, does not bleed onto adjacent surfaces, 
and gradually develops a softer green appearance. 



Discussion 

G. M. Ugiansky: In light of the information given this morning by Prof. Pourbaix on the 
stability of phases using potential -pH diagrams, I would like to know if you have considered 
the ternary Cu-S-HjO potential-pH diagram in formulating your process for artificial 
patination of copper. If the above-mentioned diagram is considered, one could theoretically 
put the Cu on the proper pH, potential, and S species concentration to produce the wanted 
patina. 

D. C. Hemming: I may have skipped too quickly over the point where I mentioned that this 
patina was identified by x-ray diffraction as brochantite which has the copper sulfate and 
the three copper hydroxides. So that would put it in the proper range on Pourbaix 's 
diagram for brochantite. 

G. M. Ugiansky: What form is this patina? 

D. C. Hemming: As near as we can determine by x-ray diffraction, it is the same as the 
natural patinas. It is basic copper sulfate. 

G. M. Ugiansky: If it is a sulfate, then one would have to have a diagram that is not 
just copper-water. It would have to be the copper-water-sulfur system. A ternery diagram. 

D. C. Hemming: I believe that it was covered on the Pourbaix diagram that was shown 
earlier for copper. 

G. M. Ugiansky: Thank you, I may have missed that part. 

D. C. Hemming: Our part in this was to attempt to commercialize a process that was developed 
at the University of Utah and we did not feel that we should digress too far from that 
which they had given us to work with. There could be other systems developed, I am sure. 

U. Bertocai: You have shown that during the formation of the patina, CU2O is first 
formed. Since the stability, as well as the electric potential, of CU2O is affected by 
light, have you ever detected any effect of illumination on patina formation? 

D. C. Hemming: We have tried that on a bench scale and could not detect any difference. 
Perhaps, we did not use the right wavelengths or intensities. 



102 



BETA IRON OXIDE HYDROXIDE FORMATION IN LOCALIZED 
ACTIVE CORROSION OF IRON ARTIFACTS 



F. Zucchi, G. Morigi, and V. Bertolasi 

University of Ferrara 
Institute of Chemistry 
Ferrara, Italy 



As a part of a research program on the restoration of buried iron artifacts, we have 
recently examined a series of iron objects of a type which was already known to be very 
difficult to restore. The objects in question were three spearheads and two axes found in 
a gallic tomb of the 4th century B.C. at Pianetto, Fori i , Italy. All the objects are 
heavily corroded, but with a substantial iron nucleus. The thick layer of iron oxides 
embedding the ground silicates is not uniform, owing to the presence of typical aggregates 
of corrosion products; the aggregates are fine-grained and brighter than the surrounding 
oxide layers, as can be seen in figure 1. These are points of active corrosion. In fact, 
even if the corrosion products of these zones are removed during the restoration, they 
win develop again by simple exposure to humid air. 




Figure 1. Aspect of nonuniform layers of iron oxides. 
The fine-grained and brighter aggregates consist of 
B-FeO(OH). 



We thought it would be of some interest to establish if the corrosion products of the 
active zone were systematically different from those of the surrounding regions. For each 
object, two different samples were drawn: the first from the active zones and the other 
from the surrounding oxide layer. The composition of the samples has been determined by 
x-ray diffraction using a diffractometer and CoK radiation. The different compounds were 
identified by comparison with the cards of the Powder Diffraction File published by the 
Joint Committee on Powder Diffraction Standards. 



103 



The results of the analyses are summarized in table 1. They show that the only 
significant difference between active and inactive zones is connected with the presence of 
beta-iron oxide hydroxide (B-FeO(OH)). It is systematically present in the active points 
and absent in the inactive ones. This result can be interpreted in terms of what we know 
about the formation of B-FeO(OH), which, unlike a- and Y-FeO(OH), is not a common corrosion 
product on iron. 

Table 1. Relative components found in the active and inactive 
region of fine artifacts. 



Artifact 1 

Zone a i_ 

Goethite a-FeO(OH) + ++ 

Akaganeite B-FeO(OH) ++ 

Lepidocrocite Y-FeO(OH) (-) 
Magnetite Ve^O^ ++ 
a-Quartz Si02 



2 3 4 5 

a i a i a i a i 

+ ++ ++ ++ + ++ + ++ 

++ + ++ ++ 

(-) (-) + 

+ ++ (-) -H- (_) ++ + ++ 
++ ++ 



Notes: a = sample drawn from the active region. 

i = sample drawn from the inactive region. 

++ = component present in large amount. 

+ = component present in medium amount. 

- = component present in small amount. 

( ) = uncertain identification owing to the peaks overlapping. 



3-FeO(OH) has been found in the rust layers of steels exposed in marine atmosphere 
[1-4]^ or submerged in sea water [5]. Keller [6] proved that a prerequisite of the for- 
mation of 6-FeO(OH) was the presence of Cl~ ions and that it is formed in the presence 
even of small quantities of CI" ions. If these ions are localized in small areas, the 
remaining metal surface will be covered by a- and Y-FeO(OH), while the CI' ions are fixed 
in the lattice of 6-Fe0(0H) already formed. In contact with metallic iron, e-FeO(OH) has 
the effect of accelerating the corrosion process because of the release of Cl~ ions. The 
role played by CI" ions in the corrosion of iron in different environments has been dis- 
cussed by Pourbaix [7] and Feitknecht [8]. 

Although detailed studies of the relationship between CI" ions and formation of B- 
FeO(OH) are not available for the underground corrosion of iron, we think the results 
obtained in other environments can be safely extended to this type of corrosion. That is, 
the presence of B-FeO(OH) is, by itself, sufficient to say that a corrosion attack is in 
progress on the metal and that such a process is caused by chloride ions, which have to be 
removed at restoration time to stop the active corrosion. 



References 



[1] Sugawara, H., Takano, M., Ebiko, H., Hashimoto, K. , Suetaka, W., and Shimodaira, S., 
J. Soo. Mater. Sci., Japan, ]]_, 710 (1968). 

[2] Misawa, T., Kyuno, T., Suetaka, W., and Shimodaira, S., Covros. Sci. 11, 35 (1971). 



Figures in brackets indicate the literature references at the end of this paper. 



104 



Misawa, T., Asami , K., Hashimoto, K., and Shimodaira, S., Cowos. Sai. 14^, 279 
(1974). 

Misawa, T., Hashimoto, K., and Shimodaira, S., Corros. Sci. 14^, 131 (1974). 

Mollgaard, J., Extended Abatraats of 5th International Congress on Metallic Corro- 
sion, p. 335 (Tokyo, 1972). 

Keller, P., Werk. u. Korr. 20, 102 (1969). 

Pourbaix, M. , Proceedings of 4th European Symposium on Corrosion Inhibitors, p. 674 
(University of Ferrara, Italy, 1975). 

Feitknetcht, W., Chimia, 6, 3 (1952). 



105 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



THE CURRENT STATUS OF THE TREATMENT OF CORRODED METAL ARTIFACTS 



R. M. Organ 

Smithsonian Institution 
Washington, D.C. 20004 



Definitions and Limitations 

First of all, the title: It would be good, of course, if you could define all of 
these words for yourselves, so that you would really know what they mean. I will try to 
short-circuit that, for the sake of saving time--The Current Status of the Treatment of 
Corroded Metal Artifacts, by which we mean the present-day standing of methods of dealing 
with artifacts. Those artifacts, of course, are products of human skill--fabrications of 
particular materials. 

We are concerned now with metal materials: lustrous, malleable, electrical con- 
ductors. Some of you may be smiling at the thought that your ancient objects satisfy that 
description! There will be some restrictions on my subject matter, quite obviously, 
partly because what we are supposed to be having here is a dialogue. Dialogue, as you 
know, means conversation across or through, from one person to another. And the dialogue 
is supposed to be among three groups. Museum conservators are one (museums, according to 
my dictionary, are places, buildings, for the storing and the exhibition of objects). 
Conservators are people whose skill keeps things together. (Sometimes we do not quite 
succeed! ) 

Another terminal in this dialogue is the archaeologist. Are there any archaeologists 
in the house? ... Did someone put up his hand? Good! We have one archaeologist. The 
burden upon you is grave. This is very disappointing: we ought to have many archae- 
ologists here. Archaeologists are defined as the scholars of prehistory but, nowadays of 
course we have to add the historic archaeologists, who are scholars also of written-down 
history. They study fabrications also in words, whereas the earlier archaeologists study 
fabrications solely in materials. Maybe the historic archaeologists by comparing the two 
may discern the truth. 

Then we have the corrosion scientists, whom we all learned about yesterday. They 
know all about the techniques but they may not know the constraints that operate within a 
museum, so I hope to say a little about that also, because some of the things that we CAN 
do we MAY NOT do. 

Another restriction upon my talk concerns the age of the artifacts. If they are to be 
archaeological then they must be of some antiquity and I think in North America that means, 
say, something over 100 years old. So it is not so very limiting after all, but even at that 
antiquity the lustrous metal objects are probably no longer very lustrous, not when they are 
first seen. They are crusted over and often penetrated by alteration products--the corrosion 
products that have formed upon the metal. Often they are vary poor electrical conductors 
also, which affect some of the things we should like to do with them. 

The third constraint upon what I want to say is that while we are all experts here, 
our expertise is in various and different disciplines. Our hosts from the Bureau are 
experts in their own right: that is why they are here. And the rest of us, by the late 
John Gettens's definition, are experts because we are visitors--experts are people from a 
long way away. 

Now just a little more clarification before I get into the technicalities that really 
interest you. Within the museum also there is a dialogue, or there should be. The dialogue 



107 



in the museum starts with the archaeologist or the curator of the archaeological objects 
or with a curator of some other objects from the collections. He is one terminal in the 
dialogue. And he should be, I think, responsible for the collection even though he is 
primarily a historian by definition. It would be lovely if some of those historians were 
here to find out the real nature of their things. The archaeologist has in the museum to 
talk to the conservator and also, if there is one, to the scientist who can help him with 
his analyses. So, similar to ours here, we have another triangle within the museum. Now, 
I would like to put drawings on the board to illustrate all these concepts and I would 
like to draw a triangle showing archaeologist-or-curator , conservator, and scientist at 
all three vertices. But then I wondered, "How should I link them together?" Should the 
links between them be in compression--a great headache all the time from all their inter- 
actions--or should they be in tension--another kind of interaction among the parties. But 
I think perhaps we ought to draw them just as struts, being mutually supportive. I was 
hoping that many archaeologists would be here today to discover how they could be supported 
by conservators and scientists. However, that is not to be. 



The Archaeologist's Need 

Now the archaeologist or the curator is really the person responsible for our problems 
he caused the transfer of the object from the ground, in this case (fig. 1) a bronze 
mirror lying in the ground at Holcombe in Devon, England; or, he is responsible for trans- 
ferring things from underwater (fig. 2)--(here is something that Lars Barkmann will tell 
us more about probably, later in the day); or, he is responsible for transferring something 




108 



Figure 3. 



Household utensils displayed. 




Figure 4. Chinese bronze from Freer 
Collection. 



from grandmother's attic, or from somebody's sale room. So, he transfers things from one 
corrosive environment into another one, namely, the museum, or his display cases. Some- 
times he wants to display these things in a social context (fig. 3), so that the kind of 



109 



appearance they have--you will realise that in a museum, display is almost always visual 
and appearance is of fundamental importance--the kind of appearance that the curator (and, 
of course his assistant display-artists) wants differs from object to object, from case to 
case. Sometimes examples of the very same object may be required in different conditions. 
Here for example (fig. 3), are some things assembled, partly cleaned but most of them 
looking soiled, rather as they may have appeared while in use. On the other hand, the 
curator may wish to have his object regarded as an art-thing in its own right. Here is a 
Chinese bronze from the Freer Collection (fig. 4) beautiful as an art form, not only 
because of its form but also because of the corrosion products that have accumulated on 
its surface and have given it its particular color. 



The Conservator's Work for the Archaeologist 

Thus the conservator has to be able to take an object, perhaps an excavated one, to 
free it from soil and then, in that condition perhaps, to stabilise it so that it will not 
change further at all, realizing that once an object enters the museum it has to be kept 
indefinitely. Ideally, it should assume some displayable form and then be left in that 
form, unchanging. So how do we keep a thing which is in an environment that is necessarily 
corrosive--almost all environments are corrosive to some degree--how do you keep it in 
that particular form? That is one of the problems that a conservator may face. 

On the other hand he may be required to take an object and not only clean it from loose 
soil but to go further to get it into some specific condition and then, again, to stabilise 
it so that it stays like that forever. This is not, of course, entirely an object-related 
technical problem because part of the environment of the object in a museum is people — all 
sorts and all kinds--with all the associations of people like clothing, dust, dirt and odors 
and this, that and the other. But I will not talk about this aspect specifically today. 

Why then does the curator choose to have an object in a particular condition? Essen- 
tially, the curator is intending to study his objects. The older curator studied his 
objects entirely visually. Here is an Isis (fig. 5) with the child Horus--an ancient 




Figure 5. An ancient Egyptian Isis Figure 6. The same Isis after being 

as received in museum. cleaned by the conservator. 



110 



Egyptian piece shown in the condition as it was received in the museum--just freed from 
surface dirt. You will notice that somebody--! regret, a curator--has taken a penknife to 
its leg, just to see what he could find. He has cracked off the corrosion crust. We hope 
he preserved it in a test tube for scientists to study later! 

The function of the conservator may be to clean that object up in order to reveal its 
shape. He may get it into this condition (fig. 6). When it is "clean" you can see all 
the decoration, for example, around the eyes. You can also see--a great delight to the 
curator--this, an inscription on its base. I cannot read the hieroglyphic but I am told 
that it reads, "Dedicated by Hor, the son of Hor." It is dated from between 600 and 700 
B.C. Made of bronze, it is about 7 inches high. 

Another reason for conditioning an object then, for keeping it on display or on the 
storage shelves--is for a more penetrating kind of study--such as the more modern curator 
indulges in--a scientific one. He may be interested in corrosion products, as we are. 
Figure 7, shows a flake taken off a piece of bronze furniture from Nimrud, a complete 
cross-section (not very well polished but sometimes this is not possible in the time 
available). Under crossed polars the corrosion products present in that quite beautifully 
layered structure are clearly visible. 




Figure 7. Corrosion products are 
studied in a flake taken off a 
piece of bronze furniture from 
Nimrud. See color plate 1. 



I 

Some curator might be interested in that aspect of an object, or he may want to look 
at figure 8 to consider its macro-structure. This is a bronze, from Igbo in Nigeria [1]^. 
It has beautiful rope-work, cast in bronze, on the surface. The archaeologist was particu- 
larly interested in finding out how it was made. So we were allowed in this case to cut a 
little out--very unusual this. Figure 9 shows this cross-section through the rope which is 
cast in one with part of the body. The left hand side of the section in figure 9 is cast 
as a second part of the body. Such a micro-structure may be of great interest to students 
of materials. 

Curators may also be interested in analyses of different materials. Figure 10 shows 
a typical analysis selected from a book [2], of the elements present in iron objects. You 
will notice in the list of elements sought, items like phosphorus, copper, and chromium, 
all of which might appear in inhibitors that we might want to apply to our objects while 
conserving them. 



1 

Figures in brackets indicate the literature references at the end of this paper. 



Ill 



Figure 8. Bronze from Igbo in Nigeria. 



Figure 9. Cross-section from figure 8: 
on right-hand side section through the 
rope which is cast in one with part of 
the body. 



Analysis of Iron Objects of the Dark Age and Early Medieval Period (%) 



Ref. 


65 


65 


21 


65 


11 


12 




Axe 


Axe 


Piece 


Arrowhead 


Bloom 


Bloom 




R.Kennet, 


Stratford 


York 


Wood- 


Down- 


Fermanagh 




Reading 


(Essex) 




eaton 


pa tri ck 






6th-7th 


9th cent. 


Saxo- 


Med. 


13th cent. 


Med. 




cent. 




Norman 








C 


0.23 


0 .049 




0.05 


0.08 


0.05-0. 47 


Si 


n. d. 


0.04 




tr. 


0. 16 


0.01 


Mn 


0.01 


0.01 




tr. 


0.02 


0.01 


S 


0. 008 


0. Oil 






0.038 


0. 014- .019 


P 


0.13 


0. 445 


0.17 




0. 061 


0.063-0.081 


As 


0.049 


0.042 




0. 008 






N 


0. 005 


0.0085 










Cu 


0.01 


0.01 




0.01 




0.01 


Ni 


0.02 


0.07 




0.004 




0.01 


Ca 












0. 005 


Mg 




0.005 




0.003 






Al 




0 .04 




0.01 




0.01 


Cr 


n. d. 


nil 




tr. 




0.01 


Mo 


n. d. 


nil 




nil 




0.02 


V 


tr. 


nil 




nil 




0.02 


Ti 














Slag 










7.25 





Figure 10. Analysis of iron objects of the Dark Age and 
Early Medieval Period (in percent) [2]. 



112 



Conservation means keeping the object together--at various levels of achievement: 
keeping it together physically, so that it does not fall to pieces and be broken so that 
nobody can study it; keeping it together chemically, so that if the curator becomes very 
interested in its analysis he will have the intact piece, not changed by application of 
chemicals. Ideally, we should like to keep it, as it is, forever. 



There are stories of an archaeologist who had analyses made of Roman bronze and found 
amounts of chromium in it although chromium was not recognized until the 18th century. 
This happened presumably because a conservator had been treating the object in an electro- 
lytic bath with stainless steel electrodes. Some chromium from the steel had become trans- 
ferred to the object. This is a story: I do not have any reference to its publication! 

Although we do not wish to confuse future scholars, we do want to clean up the object 
sufficiently to be able to study it. The situation is something of an impasse for the 
conservator; what does he do? How responsible is he? To help us with these problems we 
have a code of ethics in the museum [3]. This was written originally for art objects but 
we like to use it for any kind of object if it is unique, as is an art object. I will not 
bore you with the entirety of the code but: one, we must have respect for the integrity 
of the object; two, we have as far as possible to follow the principle of veveTsibility . 
In other words, what we do now we ought to be able to undo at some distant time in the 
future, maybe a hundred years hence; maybe a thousand years hence. Reversibility is not 
always practicable with some of the materials we have to use on objects when we reinforce 
them. Three, we have to be very careful not to confuse someone. The phrase is, "he (the 
conservator) can be expected to apply little or much restoration but he cannot ethically 
carry this to the point of deceptively covering or modifying the original." Four, it is 
also the duty of a conservator to continue to refresh and enlarge his knowledge and skill 
(that is why we are here!) and so on. Thus, there is a Code of Ethics which guides us in 
our activity. Usually we leave the curator to assume ultimate responsibility because it 
is only he who knows exactly what is the function of his object in his particular collec- 
tion. 



A conservator therefore needs to have at hand an enormous number of treatments from 
which to choose one that will satisfy some of these difficult conditions. I propose to go 
through the various treatments that are available--not all of them, but the more relevant 
ones--for objects of silver, copper, lead, tin and iron, roughly in that order, which is 
roughly the order of the electrochemical series. 

Silver Objects 

Silver is usually alloyed but sometimes it is base-alloyed with so much copper that, 
while it is still white, it is almost 50/50 copper/silver. 



Unpermitted Treatments 



Treatments Available 




Figure 11. Iphigenia Cup, 
dates from about 0 B.C. 



113 



Figure 11 shows the Iphigenia Cup. It dates from about 0 B.C. Most interesting, it 
is 2000 years old but clean! You may wonder how it got that way! Our treatment for an 
object like that would be to keep the air around it pure--keep it free from H2S so that 
the silver will not tarnish, because if silver tarnishes it has to be cleaned and that may 
rub something away. Thus, we are interested in purifying the air, using things like 
molecular sieves to absorb tarnishing gases. Or we might be interested in putting an 
inhibitor on the surface of the silver invisibly. Or we might put a lacquer on the surface, 
also invisible, but it would have to be non-cross-linking so that we could take it off 
again in the distant future. An ideal treatment for an object like this cup is benign 
neglect\ Instead of treating the object, give great attention to its environment. The 
less we do to an object, in general, the better, if it is to be preserved indefinitely. 

If the silver had been tarnished, treatments normally used would have been dissolution 
of the tarnish in one of the Dips (the acid-thiourea mixtures) or possibly electrochemical 
reduction. One would not really want to rub away an object of so great an age with rouge. 

Figure 12 shows another example, a little farther advanced in corrosion. Again 
sil ver--although it may not look like it because it was base silver--from Enkomi in Cyprus 
[4], date, supposedly 14th century B.C., it was crusted over with carbonates. This is a 
hand-colored slide: we did not always have color photography available! 




Figure 12. Base silver from Enkomi in Cyprus Figure 13. Base silver object in figure 12 
probably 1400 B.C (see color plate c). after cleaning (see color plate d). 



Cleaned, we see (fig. 13) results obtained by the use of dilute formic acid. The 
crusts on that object developed because the copper in the base alloy diffused out of it 
and was fixed on the surface as copper carbonates. Thus, the surface of the metal object 
was left unchanged--we still had the shape of the object and that unchanged shape is now 
less rich in copper than it was. The surface of the metal is purer in silver--it is 
whiter than originally. The carbonates could probably have been cracked off very easily 
and stored if we had wanted to keep samples. The surface made visible by treatment is 
sometimes called the epidermis, in other words the original surface. We shall talk about 
that later. The process of getting down to it is sometimes called cleaning. There is 
another process called treatment which goes below the surface, below the epidermis. These 
usages of words derive from A. France-Lanord [5]. 

Figure 14 shows another silver object, from St. Ninians Isle in the Shetlands [6], 
dating about 800 A.D., crusted over again with copper carbonates, because the metal was 
base silver. This had been in a different environment--not so old as the Enkomi Cup--only 
twelve centuries--yet it has corroded so far that great areas have crusted away and 
vanished. Nowadays, conservators can replace the losses with a synthetic resin, clean up 
the surface to its original appearance, and present it thus (fig. 15). 

In fact, this restored shape is still highly corrodible and if it is left in a damp 
atmosphere it will turn green. It must therefore now be stored perpetually in an arti- 
fically dried environment. There is nothing much else a conservator can do about this 
because the residual silver is held together only by the wax-like mineral, nantokite, 
cuprous chloride. 



114 



Figure 14. Silver object from St. Ninians Figure 15. Silver object in figure 14 after 

Isle in the Shetlands, dating about 800 A.D. conservator has replaced losses and cleaned. 



In order to learn what really happens inside such objects, one has to take cross- 
sections. These are not normally permitted. 



Cross-sections of Mineralized Silver 

Figure 16 shows another bowl, this one from Ur. Again made of silver, it dates from 
about 2500 B.C. It is grey in color because it is crusted all over with silver chloride. 
It has lost an enormous area where corrosion processes have gone all the way through, and 
there is no metal remaining and no crust even. This sort of situation does enable the 
conservator to provide material for a cross-section because if he has to replace all of 
the missing area he may just as well take a little more off: the curator is usually 
perfectly happy to allow this. 

Figure 17 shows a cross-section taken from an edge of one of the losses. You may 
observe the remaining bright shiny silver. You also see crusts on both sides. The crusts 
consist of silver chloride. There are one or two things to notice especially about this. 
First, the epidermis, the original surface, is preserved, although there is a great crust 
on both the outside and the inside. Also, there is mineralisation present: silver chloride 
has penetrated within the metal--you see the patches of grey. Notice also that we have sub- 
stantially a mirror image outside, in the crust, of the area inside that has been penetrated 
by mineralisation. So, if the conservator values this observation made of a cross-section, 
then he knows that if he sees a great wart on the outside of an object, then it is very like- 
ly that just beneath that wart the metal is particularly fragile. But the epidermis is still 
present! So there is still hope for restoration, for so-called cleaning. 




Figure 16. Silver bowl from Ur, dates Figure 17. Cross-section taken from an 

from about 2500 B.C. edge of a loss of the bowl in figure 16. 



115 



This phenomenon of the mirror-image must happen because some silver has diffused out 
of the metal to form the crust on the outside. It is interesting that the total volume of 
the brown crust, the silver chloride, is about twice the volume of the original silver-- 
the mass inside the metal has about the same area as the mass outside the metal. This 
reflects the fact that the density of silver is 10.5. The density of silver chloride is 
about half this, 5.5, so we now have the silver redistributed: half of it has stayed 
inside as silver chloride, half of it has diffused out and become silver chloride. This 
is an interesting point. Notice also that it has diffused out almost vertically--that is 
how we form a mirror image. 

Now we could as conservators expose that original surface, that epidermis, as shiny 
silver. You will realise, if we could crack off the crust from the outside and look at 
the surface revealed, we should then see silver: it would be bright except at places 
where silver chloride had formed inside. 

How can we do this practically? We can dissolve away the outer crust with suitable 
solvents such as ammonium thiosulphate [7], which I have used in the past because it is 
available freely in the photo shops, as rapid-photo-fixer, but in Russia they have different 
systems of supply and there they have used ammonium thiocyanate [8], which they find to 
react faster than ammonium thiosulphate. 

Figure 18 shows a buckle in a corroded condition. It is made of silver and inlaid 
with gold and niello, Saxon, about 7th century A.D. After careful treatment with ammonium 
thiosulphate on a glass-bristle brush it comes out as shown in figure 19. 




Figure 19. Silver buckle shown in figure 18 after 
treatment with ammonium thiosulphate in a glass- 
bristle brush. 

Sometimes we wish to expose detail which is finer than is present here, where the 
decoration is comparatively coarse and it does not matter about using a fine glass bristle 
brush (which scratches slightly) on the surface. In contrast, Mtihlethaler [9] in Zurich 
had a more difficult situation: he had Roman spoons (fig. 20) from Kaiseraugust, dated to 
4th century A.D. He wished to clean them in order to discover detail. He used a Dip--one 
of the thio-urea/acid mixtures. In order to avoid altering the surface mechanically he 
used ultrasonics and was able to expose all of the details in a soldered joint (see fig. 
21). Here are visible the different metals and structures present where the cup of the 
spoon was soldered onto the handle. 

There is another method of removing silver chloride other than by dissolving it and 
brushing the solution and debris away. Figure 22 shows a situation where there is a very 



116 



10cm 

Figure 20. Roman spoons from Kaiseraugst, dated 
to 4th Century A.D. 




Figure 21. Detail in a soldered joint 
of Roman spoon in Figure 20 showing 
the different metals and structures 
present. 



fine silver cage with a blue glass beaker blown inside it [10]. The cage is crusted over with 
silver chloride. It is Roman, about first century A.D. and the problem is to remove the soil 
and the silver chloride from the outside without damage to the glass. The problem was solved 
by using an electrolytic method, making the object the cathode in a solution of formic acid 
and electrolysing it at a fairly low current density, about 30 to 50 mA per square decimeter, 
until the silver chloride on the outside was converted into metallic silver, using a carbon 
anode, although stainless steel can be used less satisfactorily. Figure 23 shows the result. 
It was so good that the spin marks caused while making the silver cage can still be seen. 




Figure 22. A fine silver cage with a blue Figure 23. Silver cage shown in figure 22 

glass beaker blown inside it, Roman, after treatment by electrolysis, 

about 1st Century A.D. 



117 



There was another mark that I cannot see now, still preserved even though the object has been 
cleaned. It is clearly possible to clean without losing information. 

In this example the silver chloride on the outside was converted to silver in a form 
that could be brushed away. We can also use another technique of making the outer mineral 
crust into coherent metallic silver using cathodic reduction in caustic soda or sodium 
carbonate at 30 to 50 mA per square decimeter. Figure 24 shows a bowl to which this 
method was applied [11]. It is made of silver, from the Royal Graves at Ur, dating from 
about 2500 B.C. There was only a thin metallic core of silver remaining within the 
object. At the rim (fig. 25), thickened by the maker's skill, there was no silver at all. 
So, if we had been incautious we could have lost that information. The result is shown in 
figure 26. We have been able to convert silver chloride back into metal right up to the 
edge, where there had been no metal at all, yet the crust outside the epidermis could 
still be brushed away in order to recover the original shape and appearance. The photo- 
graph shows the stage before final re-shaping. 





Figure 24. Silver bowl from the Royal 
Graves at Ur, from about 2500 B.C. 
before restoration. 




Figure 25. Diagram of the rim of the 
bowl shown in figure 24, where there 
was no silver. 



Figure 26. Bowl in figure 24 showing 
the silver chloride replaced by 
metallic silver up to the rim of the 
bowl, using the cathodic reduction 
technique, using fully rectified 
current. 



118 



The structure of silver which has been recovered electrolytical ly is of some interest. 
Figure 27 shows a cross-section of another piece of silver after regeneration. The object 
was recovered from the same dig at Ur. The recovered silver is porous. This reflects some 
of the things we were told yesterday about the changes that can be made to corrosion products. 



Figure 27. A cross-section of a 
silver object recovered from 
the Royal Graves at Ur, (about 
2500 B.C.) after regeneration. 
Note the silver is porous. 




A further stage of treatment in handling silver chloride arises in the situation 
where there is no metal left at all. Then, one can make a coherent and polishable silver 
by a cathodic reduction technique in weak sodium hydroxide at a low current density (15 mA 
per square decimeter) but in partially rectified current. 

One object to which this had been applied [12] was the lyre from Ur (fig. 28), of the 
same date as the bowl. The photograph shows its state as found in the museum, only rein- 




119 




pared with the size of the pores in 
figure 27. 

Figure 31. The lyre shown in figure 28 
after complete restoration. See color 
plate i. 

forced since the time of excavation. It is in a terrible condition. No one was even sure 
that it had been assembled properly. The curator was very insistent that he could not 
continue to show this, partly because it was falling to pieces in the display case, and 
partly because it did not look like silver. 

Figure 29 shows an example of the process in which you can see that the grey silver 
chloride is being converted into two spreading patches of bright metallic silver, just by 
laying a thin silver wire on the surface into close contact with the silver chloride, 
holding it down by insulating polymethacrylate, and electrolysing. The cross-section of 
metal treated thus, using partially-rectified current, is shown in figure 30. It is much 
less porous, much more substantial, than the structure shown before, obtained with fully 
rectified current. This picture, by the way, represents actually two pieces of silver 
corroded together, each being split into two Laminae. After such treatment one can 
actually bend without cracking the metal that has been regenerated from silver chloride. 
In addition, every mark present on the lyre, including evidence for the matting that it 
had lain upon in the Great Death Pit, was preserved--very important for archaeologists. 
The only thing that was changed by this process was the grey appearance of silver chloride, 
changed back into metallic silver. The final appearance of the lyre is shown in figure 31 
as it is displayed now. Of course, there was a lot more to the whole process than cathodic 
reduction: the parts had to be reassembled in their correct locations, not in the way 
that they had been hurriedly put together after excavation for immediate display. 

There are questions about this treatment. Is it really permissible to take silver 
chloride and change it back into metallic silver? My attitude to this is that it took 
about 3000 years to change from silver to silver chloride and had then begun to fall to 
pieces. Now that it has been turned back into silver and its shape reinforced it has 
another 3000 years of life, even if we return it to the same excavation that it came from. 

The technical problem of making the change from mineral to metal has been studied 
further by Charalambous and Oddy [13]. This was reported at the Stockholm meeting of the 



120 



International Institute for Conservation in 1975. Charalambous and Oddy made very inter- 
esting pictures indeed, showing the transportation of silver from inside silver wire to 
the outside during the process of corrosion and its subsequent reconversion to metal in 
its new shape. They concluded that we do not know enough about this method, so we ought 
not to use it. In fact, I believe it is being used by Curtiss Peterson--who is in the 
audience--to treat many silver objects recovered from underwater in Florida. He can tell 
you all about it, more than I know, in fact, at question time. 

Doing this sort of thing to silver leads to other difficulties. I am reminded that 
there is a problem that we call brittle silver. This is silver that just falls to pieces 
in the hands. It does not respond to the silversmith's annealing because, among other 
things, of intergranular corrosion. Figure 32 shows a specimen like this at the bottom of 
the screen. We find that this kind of silver can be restored by sintering it just below 
its melting point underneath charcoal. That probably means in an atmosphere of carbon 
monoxide such as we were learning about yesterday. The picture shows a vessel from Nuri, 
of very early date, 530 B.C., which was falling to pieces, literally. On the left, the 
picture shows numbers of little fragments. Every time it was touched, another similar 
fragment came off. After our treatment it was possible to bend mis-shapen areas back and 
to add the missing area. On the right, it may be seen in its finished state, quite strong 
and tough again. One could, of course, have been extremely conservative about treatment 
and have taken all the little bits shown in picture No. 1 and have fixed them together on 
a reinforcing backing and supported them in register. Then we should have had all our 
pieces ready for study. The owners, however, wanted something done with it because they 
did not consider it safe to handle--as indeed it was not. This work was reported at the 
Museum of Fine Arts, Boston, Seminar, Application of Science in Examination of Works of 
Art, 1965, 131-132. 

Further work on the problem of brittle silver was mentioned in a paper by A. E. 
Werner [14], because the sintering method was found to be unsuccessful on some silver from 




As received Stages in annealing After annealing 




First stage in restoration Second stage in restoration Restored Vessel 



Figure 32. Restoration of silver libation vessel from Royal Cemetery 
at Nuri, about 530 B.C. 



121 



Pakistan. H. Barker, working in the same laboratory, found that he could strengthen 
similar silver by reducing it in hydrogen gas at atmospheric pressure for 30 minutes at a 
temperature of 300 to 400 °C--well below red heat. After this preliminary, he then had to 
apply the sintering process. 

All of the above leads to consideration of lifting silver objects, which may be 
exceedingly brittle, out of the ground. When Woolley lifted the Ur lyre he stuck it 
together with paraffin wax and burlap in order to remove it in one piece. Part of the 
difficulty that made the curator want to change the lyre as exhibited arose from this 
operation: it had not been put together correctly and no one could see how it should be 
put together properly. The laboratory treatment was needed to recover the original shape. 
I have come under criticism from several sources for doing this and you should be aware 
that the method, though practicable, is not necessarily permissible in a museum. 

The Russians have faced the same problem of lifting from the ground many beautiful, 
very thin silver items [8]. They have solved it by pouring over the object, in the 
ground, very pure ethanol and igniting it, repeating several times. Presumably this 
treatment heated the silver chloride enough to melt (AgCl melts at quite a low temperature, 
455 °C) and flow in order to consolidate the objects. Then they were lifted and treated 
subsequently by further heating in a furnace and then by cleaning the surfaces with their 
aqueous ammonium thiocyanate solution and a glass-bristle brush. 



Copper and Bronze Objects 

Figure 33 is a slide that many of you have seen before but I shall continue to use it 
because it is irmensely valuable. It represents a chisel from Jericho. The excavator 
considered its date to be 6000 B.C. but it may possibly be not quite as old as that. The 
picture shows two chisels corroded together within a crust and dropped in transit. The 
crust cracked through and exposed what was said to be a sewing needle inside: made of 
copper. The vertical line to the right indicates the location of the micro-section shown 
in figure 35. The object is shown in figure 34 after deliberate exposure to high relative 
humidity in the laboratory. At one place it turned green, at a location close to the metal 
The green material is that which curators call bronze disease, arousing horror if observed 
in their collections. 




Figure 33. Two chisels corroded to- 
gether from Jericho and considered 
to be from about 6000 B.C. 




Figure 34. Close-up of the chisels 
shown in figure 33. See color 
plate f. 



122 



Notice also that the crust is a double layer; there is an epidermis located in the 
middle of that thick crust of cuprous oxide. There are green corrosion products on the 
outside (this, by the way, was a hand-colored photograph). 

The cross-section through this was extremely informative (fig. 35)2. First, you 
see the green on the outside, carbonates, and within, the pink cuprous oxide. You see the 
epidermis as a black regular line all around within the pink. Notice also that when the 
object was dropped, part of the crust was cracked off mechanically, and very important for 
the conservator, this cracked off along the epidermis surface. Thus, just by cracking, 
one could recover the original shape. Remember the crust consists of oxide both outside 
the epidermis and inside the epidermis. 




Figure 35. Cross-section taken from 
chisel sample shown in figure 34. 



Notice also that further down between the cuprous oxide and the residual core of 
metal, there is a black layer which by test with a needle point beneath the microscope, 
may be observed to be waxy in structure: it is cuprous chloride. This is the material 
that expanded into the green powdery mass when exposed to high relative humidity. Notice 
also within the inner of the two crusts of cuprous oxide a line of some green material. 
This is paratacamite--the green bronze disease--and it happened there, presumably during 
antiquity, when some cuprous chloride was left behind as the interface between cuprous 
oxide and metal moved on inwards, leaving this, no longer in contact with metal, to be 
converted by oxygen and moisture from the air into the stable green disease. Metal is now 
present only in the middle: it is copper, not containing tin, so far as I know. 

The existence of bronze disease has, in my opinion, done great things for museum 
science. The curator considers its appearance to be an evil. In fact, if it had not been 
for bronze disease stimulating the curiosity of curators, we should probably not know so 
much about the corrosion of bronzes as we do now. 

Figure 36 shows bronze metal at the top and an incrustation at the bottom. In this 
copper-tin alloy we find a situation similar to the crusts on silver. Figure 37 shows a 
typical kind of bronze disease. On the left-hand side you can see several patches of 
light green at the edge of the crust. That is the location of the grey waxy cuprous 
chloride where, when it becomes exposed to the air, it forms the green paratacamite that 
the curator deplores. 



2 

For a color reproduction of figure 35, see R. M. Organ, The conservation of Bronze 
Objects, in Art and Technology: A symposium on Classical Bronze, Suzannah Doeringer, 
et al. , color plate V(B) p. 78), Cambridge, MA, 1970. 



123 



Figure 36. Bronze metal showing bronze disease. See color plate k. 




Figure 37. A copper-tin alloy object 
showing typical bronze disease. 
See color plate h. 



Figure 38 shows a diagram that indicates the options that the conservator has when he 
is asked to clean a copper object in this condition. There are four quadrants: the first 
possible option (upper left-hand quadrant) is to stabilise the object without changing it; 
perhaps to do something so that the cuprous chloride can never be exposed to high relative 
humidity and oxygen. Then it can never change to bright green paratacamite. The second 
(upper right-hand quadrant) is local cleaning; that is, removing the outer crust down to 
the epidermis. The lower right-hand quadrant shows just cleaning but also stabilisation 
by some method serving to prevent bronze disease from appearing. The fourth quadrant 
represents complete stripping of all corrosion products right down to the metal and 
beyond, into any corrosion pits in the surface of the metal, so that nothing can happen to 
it again unless we do it deliberately or are careless in storing it. 



124 



♦cleaning" to 

REVE.AL SHAPE 




STRIPPINQ 



'^CLEANIKiC" -I- 
STABiLISATlON 



Figure 38. Diagram showing four options 
the conservator may choose in cleaning 
a bronze object such as is shown in 
figure 37. 



Methods 

Let us then look at methods of stabilisation appropriate to the upper left-hand 
quadrant: doing nothing except stabilising, to prevent cuprous chloride from erupting. A 
very old method due to Rosenberg, 1917 [15], is described in principle in figure 39. One 
coats the surface with a mixture of agar jelly and the moisturiser glycerol. Then, apply 
aluminum foil all over the surface and expose the object in a wet place. The chloride is 
supposed to diffuse out and attack and be fixed by the aluminum foil. The foil develops 
holes which we re-cover with foil until eventually there is no longer attack on the foil. 
The effect is supposedly to fix and prevent further reaction of cuprous chloride that is 
near the surface. The process does a little more than that. Figure 40 shows results. 
Condition before is shown on the left and after is shown on the right. As you see, there 
is a rather unpleasant crop of coppery spots. I do not believe that these photographs are 
fully representative of the technique. I should hope for a much better all-over appearance 
given a suitable corroded object. 




saran cover 

"diseased" 
object coated 
with aluminum 
foil affixed 
with agar-agar 
jelly plus 
glycerol. 

high relative 
humidity 

WATER 



Figure 39. Diagram technique used 
[15] for stabilisation of a diseased 
bronze object. 




Figure 40. A before and after view of a bronze 
object treated according to the method in 
figure 39. 



125 



A technique of stabilisation that has been much used, almost hallowed by antiquity, 
is just to immerse the object in dilute aqueous sesquicarbonate [16]. Figure 41 shows an 
object in a solution that is going green, showing that things are coming out or that 
reaction is taking place. One keeps changing the solution until no more chloride can be 
found in it by repeated tests--three years after starting it may be finished. Then one 
rinses it off and puts in in a dry place. Nobody likes the method any longer because it 
involves too much labor. This is really substantially unskilled labor and if this were 
the only objection, then the method would not be too bad. 




Figure 41. Object being stabilised 
by immersing the object in dilute 
aqueous sesquicarbonate. 



Another technique, to avoid the protracted sodium sesquicarbonate process, is the 
silver oxide method shown in figure 42 [17]. This method appeals to the craftsman: he 
likes to get his fingers on an object, to take a needle and dig the disease out--just like 
a dentist! Then one puts a little silver oxide in the cavity and makes the oxide react at 
its interface with the cuprous chloride by putting it in a damp place. This supposedly 
seals over the edge (by formation of silver chloride). On test this behaves very well 
indeed. The little spots of dark silver oxide are not really obtrusive. Figure 43 shows 
a bronze from Nimund going a little blue now as the color-film ages (you noticed that the 
hand-colored slides had not changed their color?). One cannot really notice that the 



'disease" cxcavatco 




Figure 42. Diagram of the silver oxide Figure 43. A bronze from Nimund stabilised 

method for stabilising diseased bronze. using the silver oxide method. 



126 



spots are brown silver oxide because they are quite similar to the purplish spots of 
cuprous oxide that are often exposed on the surface. Apart from the fact that silver is 
introduced where silver should not be, this is quite a reasonable technique if one has the 
manpower to do it, once the kind of man power is available that wants to do it--in fact, 
cannot be kept away from doing it. 

Another method, later than the silver oxide method, is the use of benzotriazol e . I 
am sure you have heard about this: one just soaks the object in the solution, and nothing 
seems to happen but afterwards, magically, the object does not seem to corrode anymore. 
This has been explored by many people: Madsen [18], Greene [19], Richey [20], Marabelli 
[21], and others. It works in many cases but in others it does not. We would really like 
a method that does work and always works, if we could possibly have it. We are therefore 
still looking at the problem of benzotriazol e inhibition of corrosion on copper objects. 
But you realise that the method has really been misused. It was originally devised, and 
is still used in industry, for the prevention of corrosion of bare metal. But here we do 
not have bare metal--we have millimeter thick crusts limiting access of oxygen; we have a 
thick layer of cuprous chloride and high concentration of chloride usually requires high 
concentrations of inhibitor if they are to be effective. 

Another method for dealing with this problem is a particular electrolytic method in 
distilled water--proposed by Gettens as far back as 1936. Figure 44 is actually by 
France-Lanord but is after one by Gettens [22], late of the Freer. It shows a micro- 
section of a corroded bronze which has been made the cathode and exposed in distilled 
water at six volts against a platinum anode and has made its own electrolyte by dissolution 
of cuprous chloride, the current rising to 60 mA in two days. By this treatment the 
cuprous chloride and other copper minerals within the metal have been reduced to the white 
line visible between the crystal s--representing metallic copper. Here the process has 
been applied to a ready-prepared cross-section. It is not what we should have in the 
museum, applying it to a whole object. It does not show, what we ought to see, a cross- 
section through the whole object after it has been treated. It does, however, give an 
idea of what ought to happen. This kind of stabilisation is not supposed to alter the 
mineral crust--the green carbonates and the cuprous oxide--on the outside. I am a little 
dubious about this. Gettens himself said that the method should not be used without much 
more exploration. It might work with particular care taken to monitor progress. 

Now, another type of treatment, see the second quadrant of figure 38, cleaning to 

reveal shape. What methods do we have? An obvious method is to crack off the crust above 

the epidermis--a method used by Ternbach and by many Italians--in which one actually 
cracks off the crust using a needle, perhaps oscillated in a Vibrotool . This is always 

permissible, because one can always stop if things appear to be going wrong. It is unlike 

a chemical treatment where you put it in and hope that you have pre-tested everything so 
that nothing will go wrong. 




Figure 44. A micro-section of a corroded 
bronze stabilised by an electrolytic 
method proposed by Gettens [22]. 



127 



Another method devised by Fink and Eldridge £23] {fig. 45) is described as an electro- 
lytic method. What actually happened has been rather lost from sight since 1925 when they 
wrote about it. They said that you wrap the encrusted object in a bare wire then connect 
the wire as a cathode. What happens then is that some electrolytic action starts at the 
wire and then spreads inwards to the object. After treatment in this way one could then 
crack off the crust. So there are two parts to this: electrolytic preparation and then 
mechanical cleaning. 

Figure 46 (see colorplate) shows a bronze we treated in this way in C.A.L.; the crust 
was very far developed--you can see the bare cathode wire lying against it as a spiral. 
It was held together by a cellulose sponge, appearing here as pink spotted material. The 
sandwich has been opened up and copper has visibly formed around the wire, just as silver 
formed around the silver wire on the object shown above made of silver chloride. This 
process was not taken to completion; the number of coulombs needed was calculated and at 
the stage shown the object had received this amount. Obviously the efficiency of the 
process was very low, because we have not converted all of the mineral. After this treat- 
ment it was possible to crack off the crust and we did find the inscription we were 
seeking. The electrolyte was 5 percent aqueous sodium carbonate and 1 amp/dm^ was applied 
for four days. 




Figure 45. Diagram of an electrolytic method Figure 46. A bronze treated by the 
[23] for cleaning diseased bronze to reveal method shown in figure 45. See 

shape. color plate q. 



Next let us consider the third quadrant, clean down to original surface and then try 
to stabilise it. The methods available for stabilisation have been described before, 
either keep the object very dry or treat with sesquicarbonate or with silver oxide with 
activation in moist air or with benzotriazol e. 

Finally, in the fourth quadrant, complete stripping is a possibility. This is 
permissible sometimes. If one has a collection of weapons for example, then the curator 
may not mind having them stripped right down. You may remember the Isis and Horus shown 
earlier. This was stripped right down to the metal. The process used in that case was a 
serial method, starting off with alkaline glyerol to take off the green carbonate. Then 
dilute sulphuric acid to remove the cuprite as a mud. Then going a little further, 
electrolysis to really remove every scrap of chloride out of the surface so that it will 
never corrode again [24], followed by intensive washing [25]. 



128 



There are other methods of achieving the result; it does not have to be a serial 
method. The object can be immersed in sodium hexametaphosphate and the copper ions be 
sequestered away [26]. One can use the disodium salt of ethylene diamine tetra acetic 
acid to sequester it away [27] or, more recently, Cejka has used this same technique in 
Czecho-Slovakia with the addition of hydrazine sulfate and ammonia to expedite the reaction 
[28]. People do use dissolution techniques. 



Other States of Mineralized Copper Alloys: Bronze 

There are other varieties of corroded copper alloys, less simple than the one described. 
Figure 47 presents an example from a Romano-British funeral pyre in Dorset, England [29]. 
The object was originally a drop of bronze melted from the deceased's accoutrements, 
incinerated with his remains. The metal melted and dripped down into the charcoal in the 
fire. The photograph shows one of the many droplets found together, all having a similar 
rounded but laminated exterior. When cross-sectioned the lower part is found to consist 
of unchanged bronze where it has been in contact with the charcoal. The copper part has 
corroded. There is here no original surface, no epidermis; there are just repeating 
layers of particular corrosion products. The cuprous chloride is again down at the bottom 
between the crust and metal; this section was exposed to high relative humidity and a row 
of white dots can be seen--they were pale green really--which indicates the location of 
the cuprous chloride. Now if anyone wishes really to stabilise this bronze he has to 
reach down to this level. Whatever technique he uses--sesquicarbonate, silver oxide, 
benzotriazol e , whatever it is--this is the level at which the source of trouble lies. If 
cuprous chloride was once present anywhere else in the crust it has already changed to 
paratacamite; it is stable, it will not change anymore. Such a corrosion situation can 
proceed to completion; figure 48 shows the shape but there is not a scrap of metal left. 
It is now all mineral; you can see this clearly at the fracture. It might be pseudomorphic 
of the metal; it might still display the shape and the metal lographic structure of the 
original. There is no guarantee of this at all in a completely mineralised bronze. From 
the foregoing it is quite clear that objects underground are not protected by their 
natural patina. The Jericho chisel had an interface that was moving down through the 
metal during some 8000 years. The presence of so-called patinas may slow down the rate of 
reaction but it certainly does not prevent it. 




Figure 47. A bronze sample from a Figure 48. Example of corrosion developed under 

Romano-British funeral pyre in the epidermis of a bronze with high tin content. 

Dorset, England [29]. See color plate n. 



129 



A similar situation occurs underwater. Figure 49 is a cross-section through a flake 
of copper sulphide, viewed under crossed polars, which came off a bronze spearhead found in 
the river. The thickness of the crust is about 4 mm (1/6 inch). It has quite a beautiful 
structure. This has been growing, eating away into the bronze, as is shown by the layered 
structure, parallel with both the outer surface and the face of the metal, showing continuing 
and steady attack. 




Figure 49. Cross-section of a flake 
of copper sulphide taken from a 
bronze spearhead found in the river. 



Now people in Moscow are interested in the protective properties of patina. Kalish 
has reported [30] to the Conservation Committee of ICOM her experiments in making artificial 
patinas. She has concluded, as a result of her tests, that artificial patinas are never 
as protective as natural ones which as you see are not very protective either. So I do 
not believe that we can rely solely upon corrosion crusts as protective agents although, 
once formed, some varieties do not change in nature but only grow thicker. These present 
a stable appearance and are therefore valuable to artists for their bronze sculptures. 



Tin Objects 

Tin as found in museum objects is usually alloyed. It is made into all kinds of 
useful things, sometimes just intended for decoration. If it is buried underground it 
grows thick films [31]. Figure 50 shows a cross-section through a coin from Malaysia. 
The total height of the picture represents 2 mm of thickness. The only metal remaining is 




Figure 50. Cross-section through a 
coin from Malaysia. 



130 



shown in part at the bottom. Everything above represents corrosion products--a layered 
structure, partly broken out during the making of the cross-section because the corrosion 
products are quite brittle and friable. Analysis showed that the crust contained some 
sulphate and a little chloride. Both sulphate and chloride are corrosion stimulators if 
they are found on tin. We did not find them in that crust by use of the microscope but 
they were shown to be present by analysis. 

Figure 51 shows an insignia or cap badge, from the 1812 war. The metal was substan- 
tially tin with a little copper. We were asked to stabilise it but there was a thick 
corrosion crust. Figure 52, made by Martha Goodway, represents a cross-section in which 
the remaining metal is identified by the scratches on it. Above it is the mineral crust. 
In between is a peculiar appearance unlike the other minerals, which has been identified 
by x-ray diffraction (Walter Hopwood) as basic stannous chloride. Here we have actual 
evidence of the existence of a corrosion stimulator, again lying between metal and mineral 
crust. It begins to look as though conservators must always penetrate to basic metal in 
some way if they wish to stabilise objects against continuing chemical change. 




This sort of thing happens to tin not only underground but also underwater. Figure 
53 shows a tin pannikin found 15 feet below the surface in waterfalls in Boundary River, 
Winnipeg, apparently lost from an over-turned canoe. Marks on the bowl show that it was 
made in London between 1801 and 1821. The pannikin was one of a stack. You can see marks 
around the side made by the next one in the stack. On the pannikin are numerous crystals, 
small black ones and small white ones. How should we clean such a bowl when requested? 
In this case we did not actually clean it at all. We identified the crystals (fig. 54), 
and found that the black ones were stannous oxide and the white ones were hydrated stannous 
oxide 5SnO«2H20, which had not been observed in so-called nature before. So we were able 
to have new mineral names assigned to them: the black crystals became Romarchite, the 
white crystals became Hydroromarchite--i?(9A/4 from Royal Ontario Museum, Archaeology Depart- 
ment [32]--and the pannikin now has to be preserved exactly as it is, as the type specimen 
of these two minerals: one way of evading conservation treatment! It is not an economical 
way: it took at least 100 times the effort to satisfy the International Minerological 
Association than it would have taken to remove the crystals! 

There are, however, other things one can do. Figure 55 shows at top left; a tin hat 
coin as received. It has been buried underground in Malaysia, dating from about the mid 
19th century or later. Made of cast tin, it was cleaned to the condition shown on the top 
right using a common electro-chemical method, surrounding by zinc granules and heating in 
dilute aqueous sodium hydroxide. It came out beautifully clean--free from all of its 
crust, but slightly bluish in col or--coated with zinc. This was taken off. I am sure 
that the corrosion experts would have much preferred to have it left on, as a protective 



131 



mmm 



Figure 53. A tin pannikin found 
15 feet below the water's surface 
in Boundary River, Winnipeg, 
Manitoba, Canada. 



Figure 54. Crystals found on pannikin shown 
in figure 53 were never observed before in 
nature. Pannikin was preserved as it was 
found. See color plate g. 





Figure 55. The tin hat coins shown at the top were recovered 
from underground in Malaysia. They are from the mid 19th 
century or later. The objects at the bottom are John of 
Portugal coins brought up from underwater. They are from 
the 16th century. 

132 



film, much like galvanising iron. However, we really could not accept the idea of giving 
back a tin object looking like zinc, so we took it off. 

The objects at the bottom are John of Portugal coins, 16th century, brought up from 
underwater. The one on the left is uncleaned, the one on the right has been cleaned 
using, this time, sodium hydroxide with magnesium filings. It has come out beautifully 
clean and has preserved its color. I am not at all sure that the magnesium did anything 
to help. It is possible that sodium hydroxide alone would have served but that is the way 
it was, in fact, done. 

There is an interesting fact about coins. You might think that, if a crust 2 mm 
thick has grown on a metal surface that is itself now only 2 or 3 mm thick, then all the 
detail would have been lost. This might be true of cast objects but it is not true of 
stamped objects. In a stamped object the metal is in some places more compressed than in 
others. It is this compressed metal that corrodes faster. So, in fact, a stamped object 
will have sharper detail in the remaining metal after corrosion than before. In this 
event the crust can be taken off in every confidence that detail will not be lost--but 
only if it has been stamped. 

Tin presents another possibility. Figure 56 shows two shoe buckles. The object on 
the left has been treated. The one on the right has not. It is made substantially of tin 
with a little silver and a little copper. It is not very old but has acquired a crust-- 
grey and brittle--on the surface, about 1/5 mm thick. The buckle on the left has been 
treated electrochemical ly with magnesium ribbon in sodium hydroxide. Gale Wever, who is 
present, did this. The process was carried out hot, for about two hours. The interesting 
thing about this object was that after the treatment and the washing we tried to put a 
protective film on the tin--we tried to form an epitaxial film which would delay further 
change. This was done by a method published by Shah and Davies [33] at the First Interna- 



tional Conference on Corrosion in 1961. This involved, essentially, reducing electrolyt- 
ically to ensure that the surface really was clean, then removing the object out of the 
electrolyte through the oxygen-rich area around the anode. We hope that this particular 
treated piece will survive unchanged longer than other tin objects. Normally, of course, 
one abrades slightly to an acceptable pewter color and finish, waxes it, and looks after 
it afterwards. 



Leaden Objects 

Lead characteristically decays in museum storage by developing a white crust of basic 
lead carbonate that falls off as loose particles. Figure 57 shows an object with cerussite. 
Lead may also develop monoxide, dioxide, sulphate, chloride, crusts in appropriate environ- 
ments. 




Figure 56. Two shoe buckles, the one 
on the left has been treated by the 
method of Shah and Davies [33]. 



133 



Figure 57. Lead object showing 
characteristic decay. 



Figure 58. A lead bulla, seal 
of Pope Paul III, showing 
corrosion by lead carbonate. 



Figure 58 shows the corrosion we hate most. Sometimes it goes to completion. This 
is a bulla--! am not sure of its date--the surface is just a corrosion crust and difficult 
to read. Often these leaden things have been saved for us by earlier conservators who 
brushed shellac varnish all over it as a consolidant. It is wonderful if they did that 
then, not knowing what else to do, because we can now recover the object. The change 
would have happened as a result of corrosion stimulants--things like acetic acid, formic 
acid, which come out of the woodwork, literally, in a display case or in a storage cup- 
board, or from the cafeteria next door, where they eat mayonnaise on their salads. This 
stimulant is fatal: all the lead objects very quickly fall into little piles of white 
powder. We hope that some conservator will have spotted the condition in time and have 
sprayed it with shel lac--which is a safe varnish--in order to hold the powder together 
until we can deal with it. 

The kinds of treatments available are fairly numerous. They aim at cosmetic results: 
they must remove the crust. We must also remove the corrosion stimulants--the acetates 
and the formates--or else the corrosion will continue in the presence of moisture and 
carbon dioxide from the air. When we remove crusts we must also avoid leaving the lead in 
a corrodible state. There are many objects made of lead which have been treated by 
chemists who were not conservators and who knew that carbonates were dissolved by acids, 
so they immersed them in acetic acid, in vinegar, to clean them all up beautifully. Then 
two years later, they were corroding again. Now we know better! 

Actual methods of treatment: first, that dissolve the crust, are: 10 percent 
aqueous disodium ethylene diamine tetracetic acid, used first so far as I remember, by 
Kuhn [34] in Munich but there are several other sequestrants that will serve. We have 
been warned by Hannah Lane [35], working in the British Museum Research Laboratory, that 
this is not always a good method because sometimes the sequestrant creeps under the crust 
and attacks the metal directly. She also has some evidence that objects treated in this 
manner recorrode faster than objects treated in other ways. I think we need more evidence 
on that point. Subsequent corrosion is probably related to the presence of corrosion 
stimulants in the atmosphere at the time of cleaning. 

A second method of dissolution is to put the crusted-over object in hot distilled 
water, which as you know attacks lead, but here it is done in the presence of ion-exchange 
resins which are in the hydrogen (acid) form [36]. Figure 59 shows a lead medal whose 
inscription can really not be reacl--it appears to be the siege of some castle but is 
crusted over with lead carbonate. We may just immerse it in beads of hydrogen-form ion- 
exchange resin, heat it enough to boil off all the liberated CO2, to boil off all the 



134 



Figure 59. A lead medal crusted 
with lead carbonate. 



Figure 60. The lead medal shown 
in figure 59 after restoration. 



acetic acid, to boil off all the formic acid, and it comes out as in figure 60. Then we 
can dry it off and wax it and it is completed. Nothing has been done to the surface 
except to remove accretions and stimulators. But there is no protective film formed apart 
from added wax, unless one adopts one of the special techniques. These are, of course, 
possible. 

A third good method is one due to Caley [37]; it is good in the laboratory but not in 
the ordinary museum. Figure 61 shows some lead coins treated in this way. The colors 
here are pooi — the coins should look more like lead than green. The method is to use 
dilute hydrochloric acid, which in the cold does not attack lead, except for formation of 
a thin protective crust of lead chloride. Then one uses ammonium acetate, which I am not 
too happy about, but it has to be used in case there is some lead peroxide present, that 
will not dissolve in any other way. Caley has treated ancient lead in this way and has 
kept it for at least seventeen years without change in appearance, so it is a possible 
method. It does, however, require careful attention and I do not advocate its use in 
museums because we cannot give anything really careful attention, with telephones ringing 
all the time. 




Figure 61. Lead coins treated to remove lead 
carbonate using Caley's method [37]. 



135 



other methods dissolve away the crust. There is also an electro-chemical method. 
Figure 62 shows a seal of Pope Paul III, seen before in the untreated condition (fig. 57), 
that has been treated in aqueous caustic soda solution with some metallic zinc [38]. It 
comes up beautifully clean if you wash it out afterwards. However, I do not know quite 
how long this will last because there are often residues of zinc salts present on the 
surface and there are difficulties in washing them away. One has to use tap water for 
washing because distilled water corrodes the exposed lead. Therefore, we do not normally 
use this method except in the field or where this is the best that funds allow. 




Figure 62. Seal of Pope Paul III after cleaning 
using an electro-chemical method. (Figure 57 
shows this bulla before treatment.) 



Then there are methods of more controlled reduction using cathodic techniques. We 
have three of these, more, in fact. If you want to finish with a lead object that appears 
blue-grey in color then you electrolyse in 5 percent aqueous sodium carbonate at a current 
density of two to five amperes per square decimeter using stainless steel or platinised 
titanium anodes. The resulting blue-grey color is liked very much by some curators. Then 
it has to be washed. Washing is recommended initially in water, but then in very dilute 
sulphuric acid, which leaves a protective film of lead sulphate on the surface. This 
process [39] overcomes most of the problems of electrolysing lead in alkali. 

If, however, one wants a finished object dark-grey in color--you notice our aesthetic 

tastes, now--the same process is carried out but in 10 percent sulphuric acid at about the 
same current density. Then there are no problems of subsequent corrosion because the 

surface automatically becomes crusted over with an invisible thin film of lead sulphate. 




Figure 63. A bulla that has corroded 
completely. The portion on the left 
is restored; the portion on the right 
is held together with shellac. 



136 



In a situation where nothing remains of the metal--where it has changed completely to 
lead carbonate--but the carbonate is held together with shellac varnish, there is still 
something that can be done with it. Figure 63 shows a bulla. A bulla is a lead seal 
clamped around a string attached to the document and bearing impressions. The part 
clamped around the string is thin and corrodes through first. The one half on the left 
has been treated. The other half on the right is in fact completely non-metallic; it is 
just lead carbonate, held together with shellac. It can be changed into the condition of 
the one on the left by the method of consol idati ve [40] reduction which now-a-days makes 
use of 10 percent sulphuric acid as an electrolyte. A lead strip is attached around the 
crust that remains, to serve as the current-conductor, the cathode. A lead anode is used 
with a current density of 6-12 mA/dm^, leaving it for about 14 days, resisting any temp- 
tation to examine it, because if lifted out it will probably fall into fragments. Then it 
is taken out and found to be quite solid again--more porous than it was, of course. Then 
it has to be washed a little--but not too much because there is a protective film of 
sulphate on it and finished with wax in the usual way. This is a good way of dealing with 
completely lost leaden objects provided that the powder has been held together with 
varnish. 



Iron Objects 

We are all familiar with iron. Figure 64 shows a rusty old chariot wheel, just to 
remind you; perhaps to make you feel that you should go home immediately. Rusted iron 
seems to be the worst possible thing conservators encounter and some of us have literally 
tons of it brought to the door every week in need of treatment. The rust of course, 
contains corrosion stimulators--chlorides, sulphates, perhaps acetates. I do not believe 
there is ever a true original surface preserved in rusted objects. Many of my colleagues 
in Europe, however, believe that there are and that they can find them. I am not convinced 
of this at all, although a lower layer of black oxide can usually be found. Evidence for 
probable further corrosion (chemical instability) is the presence of wet brown globules of 
ferric chloride solution which appear on the surface. This has been known for a long 
time. Krause [41] recognised in 1882 that the ferric chloride content was the source of 
trouble with rusted iron that had been excavated. What treatments are available? 




Figure 64. Rusty chariot wheels 
with the rims entirely corroded. 



Figure 65 shows mechanical methods; we just grind away the thickest part of the 
crust, then prick away the thin residue. In order to provide an epidermis, one is shown 
here defined by the presence of inlay, which does happen. It is a purely mechanical 
method. 

How otherwise can we remove this crust? Well, there are many solvents for rust, none 
of them very good when the rust has been present for a few centuries: things like acids-- 
phosphoric acid, citric acid, oxalic acid, with an inhibitor present to prevent attack on 
bare metal when it is exposed. Then we have al kal is--sodium hydroxide, usually used hot 



137 



in the presence of sequestrants, such as some Versenes that act at an alkaline pH. These 
methods may be quite good because they not only dissolve the rust but may also dissolve 
away the corrosion stimulators, which, of course, we must do. 



There is also a method using tannin which is a good folk method: the early trappers 
boiled their tools in extracts of bark in order to give a nice blue-black finish which did 
not rust easily. Some people like the tannin treatment [42]. Archeologists love the 
black color, normally. In fact, iron treated by other methods is sometimes deliberately 
painted with a lacquer containing black pigment just to make it appear acceptable. Of 
course, it is useful also, because it provides some kind of partially permeable barrier 
between the atmosphere--the environment--and any remaining metal. So it is useful--not 
only aesthetically satisfying. 

If the crusts are very thick we use the electrolytic method. These alter the rust in 
a helpful manner. Sometimes it goes to a pyrophoric material which glows red-hot when the 
object is taken out of the tank and allowed to dry. Electrolysis could be good because it 
may remove the corrosion stimulators--which one really must remove. The process is 
commonly done at far too high a current density. It is usually done on dry crusts which 
are very difficult to re-wet. Both difficulties can be avoided if one knows that there is 
a problem. 

Iron recovered from underwater is best kept wet so that the liquid phase never loses 
continuity. Literally, it should be kept wet at all times and then electrolysed at a very 
low current density in a weak sodium carbonate solution, usually with stainless steel 
anodes in the hope that one will really get down to the location of the chlorides. This 
might require months and months of treatment and testing. 

The shape of iron objects can be recovered in ways other than mechanical. An early 
method is due to Rosenberg [43]. He took his rusted iron object and wrapped it carefully 
with iron wire, being particularly careful to locate the wire at places where there were 
pustules of ferric chloride. On top of this he applied thick wet asbestos, which was 
intended to serve as a kind of mould to hold all together. This was wrapped with more 
iron wire so that the crusts present were carefully reinforced. After an intermediate 
stage of drying the whole was heated to red heat, 800 °C, for 15 minutes. This should 
evaporate away any ferric chloride present in the crust. Then he plunged it into cold 
saturated sodium carbonate solution. After that he boiled it in dilute potassium hydroxide-- 
intended mostly to wash it while preventing flash rust. Eventually he was able to open it 
up and he found a beautiful shape where previously it had been a warted-over unsightly 
rusted object. Knud Holm can probably tell us more about this at question time. The 
process is probably still being carried out at the National Museum in Copenhagan. 




Figure 65. Diagram of a mechanical 
method for treating corroded iron. 



INLAY MARKS THE ORIGINAL 
SURFACE 



138 



Another method of removing ferric chloride, similar in principle, involves taking the 
object and, so-called, anneal it at 850 degrees for 8 to 10 hours in a closed furnace. 
The Military Museum in Copenhagen has been using this technique [44]. I shall not discuss 
hydrogen reduction because Lars Barkman will do that later in the program. 

The crudest method of removing chloride is to wash the object in hot water. Much 

effort has been spent in making machines to do this. These appear, so far as we know at 

present, to be doomed to only about 90 percent success. There are some failures because 
we cannot always penetrate to the ferric chloride layers. 

An advance on hot washing is to use an electrolytic method. The object is placed 
between two stainless steel plates, cathode and anode, in distilled water, and electrolysed. 
The electric field is believed to assist ions which diffuse out of the crust to move away 
[45]. One keeps changing the water and keeps it hot, at 40 °C. 

Another rather similar method, named cathodic desalination [46], involves putting the 
object inside an iron screen which serves as the cathode. Surrounding this is a stainless 
steel anode. A chloride ion which diffuses out of the object beyond the screen moves down 
the potential gradient and is lost whenever the solution is changed. Wihr, in Mainz, 
finds these two latter techniques very satisfactory, alternating the two. 

If one washes rusted iron, as some people do, in very dilute sodium carbonate, with 
the idea of moving the equilibrium of the hydrolysis FeCls + H2O ^ Fe(0H)3 + 3HC1 , in the 
direction of converting the iron salt FeCla to chloride ions, then difficulties are made 
because the sodium carbonate necessarily also forms much colloidal ferric hydroxide. 
This lies in the pores obstructing diffusing chloride. A method has been proposed using 
lithium hydroxide dissolved in ethanol , 90 vol plus methanol, 10 vol to overcome this 
trouble, the theory being that when the ferric chloride is converted to lithium chloride, 
this just dissolves in the alcohols and can diffuse away. But the theory does not seem 
to cope with the ferric hydroxide which is the real obstructant. 

Long ago in 1952, I tried to remove ferric chloride by extraction, hot, in chloroform, 
which does dissolve ferric chloride. I was never able to prove that the method was an 
improvement over others, but someone might look for a better organic solvent that would 
serve. 

The very latest idea for iron objects raised from sea water, and a very promising one 
when it works, has been published by Pearson [47], Australia. His method is to convert the 
Fe203, the red oxide, into the black oxide FesOi^. Thus, we do not really lose the shape 
of the mass. We merely change from red to black (and we like black on iron!). We hope 
that something happens to remove the corrosion stimulants as well, and it does. He does 
this by immersing the object in alkaline sodium sulphite under nitrogen gas in order to 
prevent atmospheric oxygen from oxidising the sulphite. It is done in an oil drum purged 
with nitrogen gas, changing the solution every day or whenever needed as the chloride 
concentration increases. Finally, he washes in water and fixes any sulphite remaining in 
the object with a bath of tenth molar barium hydroxide, which reacts to form barium 
sulphite and barium sulphate. The object is then dried. 

All of these processes require a final removal of water. If any metallic iron has 
been exposed by the process, trouble will arise from flash-rusting. There are various 
ways of avoiding this. Commonly, one rinses out with acetone or ethanol, successively, or 
one can use dewatering fluids. In the Smithsonian we use a popular commercial fluid 
containing a substance which is left behind to serve as a corrosion inhibitor. The use of 
de-watering fluids has great possibilities; they do not dilute the water then evaporate 
with it as an azeotrope, but actually physically separate the water from the metal by 
penetrating into the interface. If there is some inhibitor in the fluid it should be very 
effective. It has been found that on cast-iron machinery it serves better than anything 
el se. 

It is strongly urged that anyone who wishes to follow the treatment of iron should 
read a paper by Fenn and Foley published at the IIC Stockholm Conference, 1975. It is an 
excellent review of the situation that enables one to see what are the real problems. 
These are not all technical; some are problems of interpretation. 



139 



In epilogue: clearly, there are numerous methods available to conservators but there 
are also innumerable problems. Therefore, there are still innumerable needs. We have to 
choose a method, suitable on the one hand for tons of excavated objects or on the other 
hand for a single art object. Obviously, a single method is unlikely to serve for both. 
So conservators have to have in mind very clear objectives for their processes and they do 
need the support of curators because a curator has to state the nature of the ultimate 
objective; he, or the archaeologist, is the one in charge of collections. Now in order to 
help conservators to choose a path between the various boundary conditions of his particu- 
lar problem we need more methods of exami nation--many more methods of non-destructive 
testing--to enable us to discover the real situation before we decide what we can do and 
what we are permitted to do. We also need a great many simple monitoring techniques, not 
only to evaluate the objects before we start but also to tell us what is happening during 
the process so that we can satisfy ourselves that what we hoped we were doing is actually 
taking place. On many occasions at present it is not! 

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[I] Shaw, Thurston, The Making of the Igbo Vase, Ibadan 2S, 15-20 (1968). 
[2] Tylecote, R. F., Metallurgy in Ai-ahaeology , London, 1962, p. 263. 

[3] Code of Ethics for Art Conservators, IIC-American Group 1968, c/o New York University, 
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[4] Schaeffer, C. F. A., Enkomi-Alasia, Nouvelles Missions en Chypre (Excavations 
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[5] France-Lanord, A., La Conservation des Antiquites Metalliques , Laboratoire 
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[7] Plenderleith, H. J., The Conservation of Antiquities and Works of Art, London, 1956, 
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[12] Application of Science in Exancination of Works of Art, ibid, p. 126. 

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[15] Rosenberg, G. A., Antiquites en Fer et en Bronze, Copenhagen 1917, p. 87. 

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140 



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Organ, R. M. and Mandarino, J., Romarchite and Hydroromarchi te : Two New Stannous 
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141 



Shah, S. N. and Davies, D. Eurof, The Influence of Cathodic Treatment on the 
Subsequent Oxidation of Tin, 1st International Congress on Metall-io Corrosion, 
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Kuhn, H., Neue Reinigungsmethode fiir korrodierte Bleigegenstande, Museimskunde , 29, 
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Organ, R. M., Use of Ion-Exchange Resin in the Treatment of Lead Objects, Museums 
Journal, London, 53, 49-52 (1953). 

Caley, E. R. , Coatings and Incrustations on Lead Objects from the Agora and the 
Method Used for their Removal, Studies in Conservation, Z, 2, 49-54 (1955). 

, The Cleaning and Restoration of Museum Exhibits, Department of Scientific 

and Industrial Research, H.M.S.O., London, 1926, p. 33. 

Organ, R. M., The Consolidation of Fragile Metallic Objects, p. 131, in Recent 
Advances in Conservation, Thomson, G., ed. London, 1963. 

Organ, R. M. , Enquiry into Procedures for Electrolytic and Electrochemical 
Treatment of Mineralised Metal Antiquities, Pt. Ill, ICOM Cormittee for 
Conservation, Amsterdam, 1969. See also ref. 14 above. 

Krause, Verhandl. d. Berliner Anthropol Ges , 1882, p. 533; see Rosenberg, ref. 
43. 

Pelikan, J. B., Conservation of Iron with Tannin, Studies in Conservation, 
London, 2, 3, 109-115 (1966). 

Rosenberg, G. A., Antiquites en Fer at en Bronze, Copenhagen, 1917, pp. 40-46. 

Eriksen, E. and Thegel , S., Conservation of Iron Recovered from the Sea, Copenhagen, 
1966. 

Wihr, R. , Electrolytic Desalination of Archaeological Iron, Conservation in 
Archaeology and the Applied Arts, International Institute for Conservation, Stockholm 
Congress 1975, preprints pp. 189-191. 

Wihr, R. , ibid, p. 190. 

North, N. A. and Pearson, C, Investigations into Methods for Conserving Iron Relics 
Recovered from the Sea, Conservation in Archaeology and the Applied Arts, International 
Institute for Conservation, Stockholm Congress 1975, preprints pp. 173-181. 



See also: Stambolov, T., The Corrosion and Conservation of Metallic Antiquities 
and Works of Art, A Preliminary Survey, Central Research Laboratory for Objects of 
Art and Science, Amsterdam. 

N.B. Papers presented at meetings of the ICOM Committee for Conservation are obtainable from 
the International Centre, 13 Via di San Michele, 00153, Rome, Italy. 



142 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



SOME CONSTRUCTIVE CORRODINGS 



Cyril Stanley Smith 
Institute Professor Emeriti s 

Massachusetts Institute of Technology 
Cambridge, Massachusetts 02139 



As the other papers at this conference show, corrosion is generally regarded as evil, 
destructive, or at least undesirable. But, like the electrolytic couple that underlies it, 
corrosion has two sides. A glance at history shows that corrosion has stimulated much 
useful science and has been central to many useful orocesses and the making of many useful 
objects. One of its principal applications is in the prevention of corrosion, for the 
products of initial reaction, when they have the right structure, block further attack. 
Indeed, since the underlying electrochemistry is already fairly well understood, and is 
immutable, future research in the field will increasingly find opportunity in problems 
involving the microstructural , interfacial, and mechanical aspects of passivity. The un- 
resolved problems are more akin to diffusion-controlled mineralogenesis in the earth's crust 
than to basic electrode behavior. 

Corrosion can be broadly considered as the movement of interphase interfaces--its 
chemistry is that of heterogeneous systems in general. The first corrosion was the weathering 
of rocks after the primaeval formation of the earth's crust, with the accompanying redis- 
tribution of the available atomic species into new materials or new arrangements of the old, 
including the formation of beautiful landscapes and gemstones as well as the ores of useful 
metals. 

Some of the constructive uses of corrosion have historically included the following, 
listed in no particular order: 

•To obtain a solution for use in later chemical processing, as in making pigments, 
inks and mordants 

• To obtain directly a useful mineral corrosion product, such as the pigments verdigris 
and ceruse and fine abrasives such as crocus or rouge 

• To remove superficial layers of the products of prior corrosion; e.g., in pickling 
tarnished or heat-blackened metals 

• To obtain an adherent superficial layer of corrosion product, either for decorative 
purposes or to confer resistance to corrosion; e.g., the bluing of steel, the 
chemical coloring of bronze, the anodising of aluminum and the formation of alloy 
coatings by diffusion 

• To roughen or to smooth a surface for decorative or technical purposes, or, when 
locally restrained, to produce controlled designs as in etched armor, plates for 
the graphic artist or photoengraver , the manufacture of integrated circuits, and 
"chemical milling" to predetermined shapes 

• To expose the internal textures in materials for decorative purposes or to reveal 
their macro- and micro-structures for scientific study and process control 

• To obtain electric current for uses involving electrochemical, electrothermal or 
electromagnetic effects 

and doubtless others. 



143 



The earliest evidence of the use of many of these processes lies in archaeological 
objects, but the earliest literature on the chemical arts, for example, the Leyden manuscript 
of the third century A.D. and the Mappae Clavioula which began to take form early in the 
ninth century, are rich in recipes for producing color changes by corrosion [1]^. The 
Mappae tells us how to make the pigments ceruse and verdigris by corroding lead and copper 
respectively, while iron is corroded to yield crocus powder, or put into solution for making 
ink and it is etched with a copper-bearing solution to obtain a rough copper-plated surface 
as a basis for amalgam gilding. Fluxes are used to clean metal surfaces for both soft and 
hard soldering. 

The selective removal of copper from its alloys with the noble metals is a most inter- 
esting process. Today it is used, in the Western World, only in refining and to improve the 
color of solid gold very superficially, in earlier times, it was employed to obtain thick 
layers of pure gold on the surfaces of objects made of cheaper alloys. Chemically, this is 
identical with the inquartation and parting operations of the assayer--in mechanism, it is 
related to the dezincif ication of brass. For corrosion to proceed to a significant depth the 
presence of 50 atom percent or more of the baser metal is necessary. The Old World beginnings 
are uncertain. Wasteful of gold, the process could not compete with gilding processes in 
which thin layers of gold were externally applied in the form of foil or amalgam, but it 
remained in use as the basis of the alchemist's method of "multiplying" gold. A good summary 
of this and similar "alchemical" operations is given by Joseph Needham in his general dis- 
cussion of colored alloys [2]. 

It was in South America that this type of selective corrosion particularly flourished 
(fig. 1). Most pre-Columbian "gold" objects are actually made of alloys that contain less 
than a third of the precious metal, but they were given a pure gold surface after shaping by 
treatment with corrosive natural minerals, probably one of the varieties of basic ferric 
sulphate such as copiapite, Fe3(S04)itOH'13H20, mixed with salt. Known to early metal workers 
in the West as misy , ferric sulphate is almost as potent a corrodant as sulfuric acid itself. 
When used on Cu-Au alloys by the metalworkers of Peru and Ecuador it left on the surface a 
sub-microscopically porous layer some 50 to 200 micrometers thick of pure gold which was 




Figure 1. Mask of sheet "gold," Lambayeque, Peru. aa. 1100 A.D. 
The mask is actually about 35 percent copper, 36 percent gold, 
and 27 percent silver treated chemically to yield a pure gold 
surface. See Lechtman, reference 3. (Museum of Primitive Arts) 



Figures in brackets indicate the literature references at the end of this paper. 



144 



I consolidated by burnishing or by annealing to close the pores. The process, which the prin- 
[ cipal student of it. Heather Lechtman [3], calls "depletion gilding," can be identified by 
l|l the composition gradients beneath the surface of the object. The gold content increases in 
a stepwise manner, quite distinct from the sinusoidal gradient left in the metal by cemen- 
tation processes which are done hot and involve bulk solid-state diffusion. 

The enrichment of silver on the surface of silver-copper alloys is a process with some- 
what similar results, but this involves oxidation of the copper to form an external layer of 
scale which is subsequently removed by pickling. This process, known as blanching, was often 
used by mints to make debased coins appear to be of higher value [4]. 

The reaction of copper with arsenic vapor to produce silvery coatings of CusAs was used 
, in the third millennium B.C. [5]. The earliest of such reactive coatings were the alkaline- 
! glazed steatite and quartz beads produced in the fifth millennium B.C. In the third millennium 
' B.C., patterns in contrasting colors were produced on carnelian beads by local reaction with 
alkali [6] (fig. 2). 




Figure 2. Carnelian beads decorated by "etching," Indus Valley 
culture, 2000 B.C. probably from Chandhu-daro . (Museum of Fine 
Arts, Boston) The bead on the left is ca. 15 mm long. 



The production of colored oxide films on steel by heating it in air is well known and 
ancient. The fact that the kinetics of the reaction closely parallel the softening of quench- 
I hardened steel enables the color to be used to control the tempering operation with considerable 
i precision. Sixteenth century recipes describe the necessary knowledge of the two colors, the 
I appropriate red for quenching and the yellow through blue colors that characterize steel 
tools of the different hardnesses appropriate to different kinds of service [7] . 

Less well known than the colors of tempered steel are the oxidation colors produced on 
the copper alloy foils that were used to back gems in the days before they were cut to obtain 
total internal reflection. There are excellent instructions for obtaining purple, green, 
blue, and other colors on foils of various copper-silver-gold alloys that were carefully 
burnished in a dust-free room and heated over a smoke-free charcoal fire. "It is very 
admirable how on a suddain these copper rays will change into several colours: Wherefore, when 

I they have attained the colour you desire, take them off the fornace presently, for otherwise 

j they will alter into another" [8]. 



145 



The electrochemical replacement of one metal by another was noticed in antiquity. Pliny 
mentions that iron when smeared with "vinegar or alum" becomes like copper in appearance [9]. 
Replacement seems to have been used in Roman times for tin-plating bronze and it definitely 
underlies the recipes for the preparation of iron surfaces to receive amalgam gilding that 
are given in the ninth century Mappae Clavicula. One of these (Chapter 146H) reads: 

"Rounded alum, the salt that is called rock salt, blue vitriol, and 
some very sharp vinegar are ground in a bronze mortar; the cleaned 
iron is rubbed with these [materials] using some other kind of soft 
little point. And, when it has taken on the color of copper, it is 
wiped off and gilded, and then, after the quicksilver has evaporated, 
it should be cooled in water and rubbed with a tool that is very smooth 
and bright until it becomes brilliant." 

The conversion of iron objects into copper when they were immersed in certain mine 
waters was sometimes thought to be a proof of the possibility of alchemical transmutation of 
one metal into another. The Chinese writer Shen Kua in 1086 A.D. refers to a mountain 
spring whose waters contain a bitter "alum" which becomes copper when it is heated and an 
iron pan containing it is slowly changed into copper [10]. 

In the seventeenth and eighteenth centuries, cement copper was commercially produced in 
Erzgebirge. At Herrengrund the copper was unusually pure and it was used to make charming 
dishes and cups (fig. 3), often incorporating models of mining and smelting buildings and 
bearing rhymed reference to their former life as a baser metal [11]. 




Figure 3. Cup made from cement copper. Herrengrund, ca. 1700 A.D. 
The inscription reads: "Ein Pferd miah vor mit fussen trat,/ da 
idh nodh Eisen ware,/ durah ziment wassers baad/ bring ioh gut 
freund zu haare," in reference to its origin as iron in the form 
of scrap horseshoes. (Author's collection) 

The recovery of silver from waste parting-assay solutions by reaction with copper bowls 
was a standard procedure described in Ercker's great treatise on assaying [12] of 1574. By 
the early eighteenth century, assayers knew of the successive replacement of silver from 
solution by copper, of copper by iron, and of iron by zinc. In 1718, Geoffroy published his 
famed table of chemical affinities (fig. 4), which was an important step in the development 
of the theory of chemical reactions [13]. The columns listing the experimentally observed 



146 



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observes entrc dLfftrmtes substanaj 





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^r\^ Espnt-J aaAcs ^ Terrz, dbjorbarit^ ^ Cnivre, 

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0* SeLalcaL rcLauL J Arqinc PC Pitrre Calanuitaur. 

Figure 4. Table of the relative affinities between various salts 
and metals, ranked in order. Geoffroy, Mem. ^eatf. S'c-i. , 1718. 

order of the replacement of metals from aqueous solutions of their salts form what would 
today be called the electrochemical series, but it was, of course, long before the discovery 
of the role of electricity. Another landmark in the scientific study of corrosion is K. F. 
Archard's Reaherehes sur les Proprietes des Alliages Metalliques (Berlin, 1788), which lists 
the response of nearly 900 alloys to four different corrosive atmospheres. 

Electrochemical corrosion and redeposition was the basis of that pretty chemical toy, 
the arbor dianae, the tree-like growths of metallic silver produced from solutions of silver 
nitrate in contact with mercury or copper. In the 1680' s, electrolytic corrosion was observed, 
though not explained, in the loss of iron rudder fixings on some ships of the British Navy 
the bottoms of which had been coated with lead sheets from the new rolling mill at Deptford 
[14]. Though there were speculations about possible effluvia emanating from the lead and 
corroding the iron, the phenomenon attracted little attention. A century later, in reporting 
his fine early laboratory study of ancient metal objects, George Pearson [15] commented both 
on the fact that water would not corrode iron if air was excluded (a fact experimentally 
established by the French cutler, J. J. Perret, in 1772) and on the acceleration of the 
corrosion of iron that occurred when it was in contact with copper. Altogether, it is odd 
that voltaic electricity was not discovered much earlier than it actually was, and that it 
needed the intervention of a frog's leg. Evidently, a phenomenon can be well known without 
provoking curiosity on a theoretical level. 

All work on electricity between Galvani's discovery and the replacement of batteries by 
the magneto-electric generator could be claimed under our rubric of Constructive Corrosion, 
for the sacrificial solution of an anode was then the only source of current. The first 
practical uses were in the almost simultaneous inventions of the telegraph and of electro- 
plating and electroforming. Beginning around 1840, the latter quickly became of great 
importance in the arts, and it provided a popular hobby that introduced many youngsters into 
the wonders of electricity [16]. Appropriately, the first print from an electrotyped plate 
to be regularly published was one of the electrolytic cell itself which appeared in the 
January 1840 issue of Sturgeon's Annals of Saienae (fig. 5). 



PnnetptkjiuhiirffuS9^^Prvteff>e 

•rfi Esprit dcmnait^rtf 
^ EaiL . 

e 

"W" Ejpntdf vin^ £jprttf areUnls 



147 




FIGURE 1. 

Fku rk I !■< a Sccliim of Ihc necessary 
App^iraiiis, wliuli may be niailc of any size. 

(A.) An carlhcnwarc vessel, conlaiuing a 
solution of buliilinlc uf Coiipcr. 

(C.) An inncriian.of eartlienwatc or wood 
having a pla'slerof paris liuUnm, mads lo lit 
into liic interior of (a), and containing n saline 
or acidulous solution. 

(B.) The plate to be depnjiled on; im- 
mersed in thccu|)rcoiis solution, and liaving a 
wire (!•') allaclicd, wliicli tonnccLi with Iho 
bindinK strew (K), soMitimI to the zinc iilate 
(U) immersed in the saline solution. 



D 




*,* The above onfH'.ivinf: lia.s hci'ii pioiliiccd (in relief) by the Electro-chemical Process 
Oty^dis the first result of that jivocess aiipeariiii; in iirint. 



Figure 5. The first regularly-published print from an electrotype 
plate, showing the cell used to produce the plate. From Annals 
of Electricity , William Sturgeon, ed. , No. 4, 4 January 1840. 
(Courtesy Burndy Library) 



The etched plate of the graphic artist with its intaglio lines is probably the best 
known use of localised corrosion. Prints made by the process first appear around 1500 A.D. 
[17]. Leonardo da Vinci used it in 1504 and Albrecht Durer in 1515, but there was a long 
prehistory in the decorative etching of metal surfaces particularly those of steel arms and 
armor (fig. 6), but also of copper alloys [18]. The first printed book on iron and steel is 
mainly on etching them [19]. 

Although in Europe, etching was mainly used to bite in patterns previously drawn by an 
artist, in the Middle East and Far East the etching was used almost entirely to reveal the 
texture of the metal itself, thereby giving surface indications of both gross heterogeneities 
resulting from casting and forging procedures as well as the finer structural features that 
depend upon crystalline separations of the kind that, when they were later studied under the 
microscope, provided the basis of today's material science [20]. Although the internal 
structure of any material can be seen to some extent in the fine detail of fractured surfaces 
(and observations of these almost certainly prompted the first "atomic" or corpuscular 
theories of matter), the subtler details of microstructure can only be seen on carefully 
prepared sections that have been submitted to etching. 

The discovery of carbon in steel occurred in 1774, after centuries of good steelmaking 
guided by the natural though mistaken belief that heating iron in a hot charcoal fire must 
purify it [21]. One of the many results of the stimulus given to European science by contact 
with oriental materials occurred when a Swedish metallurgist came to look closely at the 
blackish residue responsible for the visible pattern on "Damascus" gunbarrels, which were 
finished by etching (fig. 7). A few decades later the same etching technique was used to 
reveal crystal structure in metals for the first time, initially in a meteorite, then to make 
fancy bibelots of crystalline tinplate, and, almost fifty years later, for the first scientific 
studies of the microstructure of terrestrial steel, from which stem modern metallography and 
much more [22]. 

It is interesting to note that the discoverers of the meteorite and steel structures-- 
Widmanstatten in 1813 and Sorby in 1864--both first published their results in the form of 
prints made directly from inked metal surfaces etched in relief (fig. 8), as Leonardo had 



148 




Figure 6. Hunting sword with etched decoration, pictured in sections 
to show detail. Made by Ambrosius Gemlich in Munich in 1540. 
(Wallace Collection, London) 





Figure 7. Etched gunbarrel with so-called Damascus texture. 
Turkish, eighteenth century. (Victoria and Albert Museum) 



149 






Figure 8. Print from the etched and inked surface of the Elbogen Meteorite, made by 
A. von Widmanstatten and C. von Schreibers in Vienna in 1813. The original is 21 cm 
high. (Author's collection) 

proposed over three centuries before. By the 1880' s, many engineers and chemists were 
studying etched metals under the microscope and relating the structures to problems of 
utility and understanding. Not the least of the consequences of the new method of study was 
a deeper understanding of the process of corrosion itself, though to this day a metallographer 
cannot help but feel that there is not enough attention paid to the microstructural aspects 
of the corrosion process. 

Finally, the intentional coloring or patination of metals in jewellery, sculpture, and 
architecture is an enormous field with a large literature, much of which is worthless. Here 



150 



Figure 9. Japanese sword guard [tsuba), nineteenth century. Several different 
alloys are combined by intricate inlaying technique and given their different 
characteristic colors by a final treatment in a pickling solution. See color 
plate p for detail of lower right corner. (Museum of Fine Arts, Boston) 
Author's photo. 




Figure 10. "Instantaneous Light Box" with case 
made of tin-plated iron, crystallized, etched, 
and lacquered. Made in London about 1820 by 
"J. Watts and Co., Chymists, No. 478, Strand" 
(Photo courtesy Bryant and May Ltd. and The 
Science Museum, London) 



151 



more than in most areas an examination of objects in a museum laboratory is more revealing 
than reading about them in a library. Not all such patinations are simple sulphides or 
oxides. Particularly in need of further study are the Chinese black bronzes of the late Chou 
and Warring States periods [23], the hot-forged high-tin bronzes of Soghdian Iran with their 
brownish black coating which seems to have been formed by oxidation prior to the final quench 
[24]; the fine black finish given to zinc alloy castings known as Bidri ware, originating in 
the Hyderbad district of India; and, towering above all these in both technical ingenuity and 
beauty, the colored metals used by Japanese tsuba makers especially in the eighteenth and 
nineteenth centuries (fig. 9) [25]. Prominent among the alloys used are shakudo and 
shibuiahi which after pickling acquire, respectively, a beautiful warm purplish black color 
and a slightly frosty dark greenish brown (fig. 10). These are alloys of copper with gold 
(ca. 5 percent) and silver [aa. 25-30 percent). Technically, if not historically, their 
ancestry includes the more gaudy finishes of metallic silver and gold that the metalworkers 
of South America many centuries earlier had obtained by corroding more concentrated alloys. 

All these corrodings, and many more, constitute an integral part of the story of man's 
discovery of the wonderful diversity of the properties of materials which he can enjoy, 
incorporate in his philosophies, and employ in innumerable devices. 



References 

[1 ] Caley, E. R. , The Leyden Papyrus X., Journal of Chemical Education, 3_, 1149-1166 

(1926); Smith, C. S. and Hawthorne, J., Mappae Clavicula: A Little Key to Medieval 
Techniques (Philadelphia: American Philosophical Society, 1974). 

[2] Needham, Joseph, Science and Civilisation in China, Vol. 5, Part 2, Section A33, 
Alchemy and Chemistry (Cambridge, 1974). 

An important source that he does not cite is Book V "which treateth of Alchemy; 
shewing how Metals may be altered and transformed one into another" in G. B. della 
Porta, Magiae naturalis lihri viginti (Naples 1589). Eng. Translation, London 1658. 

[3] Lechtman, Heather, Gilding of Metals in Pre-Columbian Peru, in Application of 

Science in Examination of Works of Art, W. J. Young, Ed. (Boston, 1973), pp. 38-52. 

[4] Boizard, Jean, Traite de Monoyes (Paris, 1696); Cope, L. H., Silver-Surfaced 

Ancient Coins, in Methods of Chemical and Metallurgical Investigation of Ancient 
Coinage, E. T. Hall and D. M. Metcalf, Eds. (London 1972), pp. 25-278. 
Similar techniques were used very early in Peru. 

[5] Smith, C. S., An Examination of the Arsenic-Rich Coating on a Bronze Bull, in 
Application of Science in the Examination of Works of Art, W. J. Young, Ed. 
(Boston, 1973), pp. 93-102; McKerrell, H. and Tylecote, R. P., The Working of Copper- 
Arsenic Alloys, Proceedings, Prehistoric Society 38_, 209-218 (1972). 

[6] Beck, H. C, Etched Carnelian Beads, Antiquaries Journal, 21, 384-398 (1933). 

[7] See particularly the sources given in translation in chapters 1, 3, and 4 

in Smith, C. S., Sources for the History of the Science of Steel, 15Z2-1786, 
(Cambridge, Mass. , 1968). 

[8] della Porta, G. B., Magiae naturalis lihri viginti (Naples, 1589). Quotation is 
from English translation (London, 1658), p. 188. 

Other good accounts of colored foils are Cellini, B., Due tratate (Florence, 1568) and 
Smith, G., Laboratory and Schools of Arts (London, 1740). 

[9] Pliny, Eistoria naturalis, XXXIV, 149, translation, K. C. Bailey, (London, 1932), 
Vol. 2, pp. 60-61. 

[10] Cited by Needham (reference 2), Vol. 2, p. 267. 

[11] Alexander, Gustav, Herrengrunder Kupfergefasse (Vienna, 1927). 

Early accounts of cement copper are: della Porta, G. B., Magiae naturalis (Naples, 



152 



1589) Vol. 4; Browne, E., Philosophiaal Transactions 5^, 1042-1044 (1670); and Schluter, 
C. A., Grundliehev Unterriaht von Hutte-Vlerken (Braunschweig, 1738), pp. 461-471. 

[12] Ercker, Lazarus, Besahreibung allerfurnemisten mineratisohen Ertzt unnd Berakwerksarten 
(Prague, 1574), English translation (Chicago, 1951), pp. 167-169, 223. 

[13] Geoffroy, E. F., Memoires de I'Aaademie Royale des Sciences (Paris, 1718), pp. 202-212. 
J. H. Westbrook, in an article to be published in Metallurgical Transactions 1977, 
shows that some of these data when placed in order against electrode potential yield 
a smooth curve. 

[14] Pepys, Samuel, Naval Minutes (1680-83), Publications of the Naval Record Society, 
60, 115 (1926); Thomas Hale, The New Invention of Mill'd Lead for the Sheathing of 
Ships Againt the Worm, (London, 1691). 

[15] Pearson, George, Observations on Ancient Metallic Arms, Philosophical Transactions, 
Read 6 June 1796, 57 pp. 

[16] For a brief history and bibliography of the beginnings of electrotyping, see Smith, 
C. S., Reflections on Technology and the Decorative Arts in the Nineteenth Century, 
in Technological Innovation and the Decorative Arts, I. M. G. Quimby, Ed. 
(Charlottesville, Virginia, 1974), pp. 1-62. 

[17] Hind, Arthur M. , A History of Engraving and Etching (Boston, 1923); Reti, L., 

Leonardo da Vinci and the Graphic Arts, Burlington Magazine, IJH, 189-195 (1971). 

[18] Smith, C. S., Metallurgical Footnotes to the History of Art, Proceedings American 
Philosophical Society 116 , 97-135 (1972). 

[19] Anonymous, Von Stahel und Eysen (Nuremburg, 1532). English Translation in Smith, 
C. S., Sources for the History of the Science of Steel (Cambridge, Massachusetts, 
1968), pp. 1-19. 

[20] Smith, C. S., A History of Metallography (Chicago, 1960). 

[21] Smith, C. S., The Discovery of Carbon in Steel, Technology and Culture, 5^, 149-175 
(1964). 

[22] See reference [20], especially Chapters 12 and 13. 

[23] Chase, W. T, Discussion presented at this conference. See this volume, pp. 191 ff. 
[24] Meyers, Pieter, Metropolitan Museum of Art, private communication. 

[25] Ogawa, Morihiro, in Nippon To. Art Swords of Japan, W. A. Compton, J. Homma, K. Sato, 

and M. Ogawa, Eds. (New York: Japan Society, Inc., 1976); Robinson, B. W., The Arts of 
the Japanese Sword (London, 1970); Roberts-Austen, W. A., Colours of Metals and Alloys 
Considered in Relation to their Application to Art, Journal Society of Arts, 36, 1137- 
1146 (1888); idem, 41, 1022-1043 (1893); Gowland, W., Metals and Metal Working in Old 
Japan, Transactions Japan Society, (London) j[3, 20-100 (1915). E. Savage and C. S. 
Smith, A Laboratory Study of Some Japanese Tsuba, Ars Orientalis, (in press). 

Discussion 

J. Kruger: You showed a Japanese work that showed the cherry blossoms floating and you said 
that it was all part of the texture. How did they do that? 

C. S. Smith: You start by taking a composite or multilayer sandwich of different metals or 
alloys. You join these by soldering and then you hammer it out to get a fairly thin sheet. 
Now if you want to produce a real pattern you make one layer of the metal a little thicker 
than the other, to allow some tolerance, then with a sharpened punch you depress part of the 
surface so that when you cut a section through it you will expose areas in the shape or 
pattern that you want. It is rather like a contour line on a map. 



153 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976. Issued July 1977. 



CONSERVATION OF RUSTY IRON OBJECTS BY HYDROGEN REDUCTION 



L. Barkman 

Wasavarvet 
National Maritime Museum 
Stockholm, Sweden 



It may well be that some are not familiar with the story of the Warship Wasa. I shall 
therefore give a brief outline of its history before dealing with its preservation. The 
Swedish King, Gustavus Adolphus, ordered the building of the ship in 1625. The Thirty 
Years' War was raging in Europe, and Sweden was striving to assert herself as a Great Power. 
The Baltic separated Sweden from the Continent and a strong fleet was essential to the 
successful prosecution of the war. 

The 38-metre keel of the ship was laid in 1626 and only two years later, in 1628, the 
Wasa set out on her maiden voyage. This ended in catastrophe after only two nautical 
miles: the Wasa capsized and sank in Stockholm harbour. The exact location of the wreck 
was pinpointed by Anders Franzen in 1956, and salvage work started soon afterwards. On 
April 24, 1961--after admirable work by divers--the Wasa broke surface after fully 333 years 
on the sea bottom. She was pumped dry and towed on her keel into Beckholm dock in Stockholm 
harbour. She was placed on a concrete pontoon and the archaeological excavation of the 
hull began. Figure 1 shows the Wasa in dry dock. The ship was presently furnished with an 
aluminum housing and in the autumn of 1961, well before the onset of winter, she was towed-- 
inside her new housing and on the pontoon--to the present Wasa Yard in Stockholm. On 
February 1, 1962, the Yard was thrown open to visitors. 




Figure 1. The Wasa in dry dock, 1961. 



155 



This massive find of waterlogged wood is the largest of its kind ever recovered. The 
vessel's displacement was 1300 tons, overall length including bowspit 230 feet, maximum beam 
38 feet 4 inches, height of aftercastle 65 feet, and draught 15 feet 5 inches. We were 
confronted with a formidable task: the preservation of a volume of 25,000 cubic feet of 
waterlogged wood with a total surface area of 150,000 square feet and containing some 
25,000 loose finds. 

Early in 1961, the Wasa Preservation Department was formed, together with a panel of 
experts, into the Wasa Preservation Committee. In July 1964, it was reorganized into the 
National Maritime Museum's Preservation Committee for the Wasa. 

The iron objects constitute the major finding after wooden objects on a volume basis. 
Most metals do not occur as pure elements in nature but must be extracted by the application 
of energy. According to the laws of nature a system strives to attain the lowest possible 
state of energy, which for metals usually means that they tend to return to their oxidized 
state and hence to their stable energy form. That iron corrodes under certain conditions is 
therefore as natural as that a stone falls to the ground. 

The extent of corrosion is to a high degree dependent on the environment. In archaeo- 
logical contexts, a distinction must be made between corrosion in a maritime environment, in 
air, and in soil. A metal corrodes if it is exposed simultaneously to oxygen and water. 
Corrosion takes place electrochemical ly, as the corrosion medium is an electrolyte. The 
dissolution of the metal is an anodic process occurring simultaneously with a cathodic 
process in which oxygen, and sometimes hydrogen ions, are consumed. Examples of the amount 
of corrosion in micrometers per 10 years for unprotected steel in different environments are 
shown below. 

Ai r Sea Soi 1 

(relative corrosi veness) 

Industrial Urban Rural River Sea Tap High Medium Low 

1000 500 100 500 1000 100 1000 300 50 



Corrosion proceeds in the soil at different speeds due primarily to differences in its 
specific electrical resistance. The lower the resistance, the greater the quantity of water 
and salts. Measurement of the resistivity therefore gives a good idea of the corrosive 
effect of a soil. Another factor affecting corrosion is the presence of oxygen. If the 
content of oxygen diminishes at greater depths in the soil, the speed of corrosion diminishes 
correspondingly. The amount of corrosion in soils thus varies according to whether, among 
other factors, they were formed in salt water, fresh water, or through weathering, and is 
much higher below than above the marine limit. In judging the corrosion of an object, 
therefore, it is important to find out whether it has been buried and in which marine 
deposit. As is evident, sea water is very corrosive. Of fresh waters, river water is more 
corrosive than, for example, hard tap water, in which the relatively high content of calcium 
provides some protection against corrosion. 

Rusting takes place in humid air when the relative humidity is above about 60 percent. 
For rusting to start in humid air, however, it is necessary that water layers adhere to the 
iron object during a long period. If these conditions are fulfilled, one may imagine the 
rusting to take place in two stages: 

First oxygen, dissolved in the water, oxidizes iron to Fe(II): 



2Fe(s) + O2 + 2H2O = 2Fe(0H)2(s) (1) 
(ferrous hydroxide, white 
or green precipitate.) 

This reaction is promoted both by impurities in the metal surface and by the increased 
degree of acidity of the atmosphere caused by, among other factors, the combustion gases SO2 
and CO2. 



156 



In the second stage, oxygen from the atmosphere oxidizes the Fe(II) further to Fe(III): 

2Fe(0H)2(s) + = 2FeO(OH)(s) + H2O (2) 
(ordinary reddish- 
brown rust). 

At the outset, therefore, the rust may be considered to consist of iron (III) oxide hydroxide 
with varying water content. For the sake of simplicity, 2FeO(OH) may be written Fe203-1H20. 
But if the available quantity of oxygen is limited, reaction (2) may be modified, leading to 
the formation of different intermediary oxidation products of ferrous hydroxide, as appears 
from reactions (3) and (4). 

3Fe(0H)2(s) + I5O2 = 2H2O + FejO,, • H20(s) (3) 

(green magnetite) 

FejO^ . H2O = H2O + Fe304(s) (4) 
(black magnetite) . 



The local accumulation of reddish-brown rust on an iron object through reaction (2) suffices 
to reduce the supply of oxygen to the ferrous hydroxide formed. It is therefore common to 
find laminated corrosion products on iron, consisting of an inner core of black magnetite, 
a thin shell of its green hydrate, and finally, an outer coating of ordinary reddish-brown 
rust. Such a coating can be produced in practice either by reducing the partial pressure, 
i.e., volume fraction, of the oxygen over the solution or by placing the objects at a greater 
depth in the solution so that the resupply of oxygen through diffusion from the liquid 
surface is retarded. In the Wasa case, the conditions on the sea-floor were undoubtedly 
sufficiently anaerobic for the aforesaid formation of magnetite to take place. 

The chlorides, mostly in the form of iron chloride, are the most dangerous enemy of 
iron. Iron chloride on most artifacts is difficult to wash out. If water vapour is present, 
it is absorbed and the FeCls solution migrates to the iron, dissolves it, and forms FeCl2. 
Under the action of oxygen from the air, iron hydroxide is precipitated and FeCls is formed 
again. Heavy destruction of the iron in the form of pitting occurs. 

Examinations of the water at the place where the Wasa sank show that the salt content 
was 4°/ooo, the oxygen content zero, and the hydrogen sulphide content 7 mg/1 in 1943. The 
temperature has varied between +1 and +5 °C. 

During the 16th century, the King Gustav Vasa had the Oxdjupet Inlet closed in order to 
prevent vessels from entering Stockholm by that route, which must have considerably contri- 
buted to a reduction of the water flow; in 1838-39, Oxdjupet was closed completely, after 
which Kodjupet became the only inlet to Stockholm. In 1867, the first clearing of Oxdjupet 
started, and in 1919, was continued to a depth of 10.5 metres. The channel was further 
deepened in 1929 to 12.5 metres. Stockholm harbour had served for a long time as a sewer 
for domestic and other waste. Everything suggests, therefore, that the conditions had 
remained the same, i.e., anaerobic with content of hydrogen sulphide. 

The conditions for corrosion of the Wasa are known, and therefore the corrosion can be 
studied under known premises. It is also of interest to study where the iron that rusted 
away has gone and, finally, how the remaining iron objects should best be treated in order 
to be preserved. Both wrought and cast iron have been found on the Wasa. Of the roughly 
800 iron objects surviving, only a score or so have been wrought iron; the rest are made of 
cast iron. 



157 



Wrought Iron 



Iron objects Upper gun deck Lower gun deck Orlop deck Hold Total 



Bolts 3 3 

Pump handles 4 4 

Capsquares 1 1 

Eyebolts 7 7 

Saws 2 2 



Cast Iron 

Pots 5 6 11 

3 pound cannon-balls 82 82 

24 pound cannon-balls 7 64 230 301 

Iron hook 1 1 

Chain shot, 24 pound 2 225 227 

Chain shot, 3 pound 12 12 

Pike shot, 24 pound 133 133 

Pike shot, 3 pound 4 4 



Of more than 5000 one-inch bolts--many are more than one metre in length--fragments 
remain of only two. All iron nails in the decks had rusted away. The original number of 
nails was even greater than that of the bolts. Other wrought iron parts which have prac- 
tically entirely disappeared are the forgings for the 64 gun carriages and also for the 52 
gunports. The speed of rusting is shown by the corrosion of the bolts which the divers 
placed in the hull during salvaging of the Wasa. At the point of greatest attack the 
diameter of one bolt had diminished to one-third in a year's time. The rapid rusting of the 
gun carriage forgings is a probable explanation of why, during the 17th century, the guns 
could be released from their carriages and salvaged with the primitive means then available. 

All cast iron objects on the other hand have been preserved, although many are corroded 
right through. Among other items preserved was a pot of 190 liters volume and 80 centimeters 
diameter from the galley. The corrosion of the cast iron objects varies. Some objects have 
practically not corroded at all, while others of the same composition and volume are corroded 
right through. This is exemplified by the distribution of the degree of corrosion of cannon- 
balls. 



Iron residue in Percentage of the total 

the balls, percent number examined (228) 



2 




3 


12. 


5 


9 


27. 


5 


9 


40 




11 


52 




17 


67. 


5 


27 


85 




17 


90 




6 



The cannon-balls were found in two stores in the hold. The outermost balls and those not 
covered by a thick layer of mud are those which are most corroded. The average corrosion of 
the balls is 14 mm during 333 years, or 0.042 mm per year. This average corrosion is thus 
lower than for the bolts which had not been covered by a protective layer of mud. The mud 
had a pH value between 2.8 and 3.2 and a specific conduction resistance of 100 to 140 ohm 
cm. 

By way of comparison the following analysis for the compact iron core from forgings and 
castings are of interest: 



158 



Amount found, in percent 
C Fe.„. Si Mn P S 



Shaft of sheave (14577) 0,3 99.1 

Bolt, 1 inch (unnumbered) 0.5 98.8 

Bolt, 1 inch (20083 1.06 <0.1 0.01 0.02 0.015 

Iron support in main mast (22014) 0.6 <0.1 0.01 0.03 0.010 

Cannon-ball, 24-pound (13698) 3.58 95 1.21 0.07 0.04 05 



The higher content of carbon, in the form of graphite, of the cast iron balls was probably 
the main cause why they have retained their volume regardless of the degree of corrosion. 
It has been possible to check this by comparing the present volume with archival data, e.g., 
for the 24-pound cannon-balls which have retained their diameter of 5-1/2 inches or 14 cm. 
Similar tests made on the bolts of wrought iron show a fairly large variation of carbon 
content. Examples of analyses of the rusty iron: 



Amount found, in percent 

Fe.„, C Si Mn P S CI 

tot 



Porous material of: 



57 


5 


2.2 






















72 


0 


15.5 


3 


51 


















43 




12.4 


5 


35 


0 


09 


0 


06 


0 


02 


0.7-1 


.2 


47 


9 


15.3 


2 


3 


0 


55 


0 


13 


0 


79 






11 


8) 
























40 


2 


15.5 














0 


15 


1 


2 


42. 


5 


15.8 


















2 


0 



Hub (14576) 
Cannon-ball (14309) 
Cannon-ball (13698) ca 
Cannon-ball (22900) 

( Fe^^ . 

Chain shot (A) 
Chain shot (A), duplicate 
test 

For the same reason, apart from the high carbon concentration, the percentage of sili- 
con has risen. This may be an additional contributory reason why the cast iron findings 
have retained their volume despite corrosion. The main element in the rusty iron is Fe203. 
The low sulphur content shows that there are few sulphates or other sulphur compounds in the 
corroded layer. The analyses show high chloride contents. According to investigations made 
by the Copenhagen T(!ljhus Museum, the chlorides exist mostly in the form of iron chlorides, 
i.e., the chlorides in the water have been converted into iron chlorides under formation of 
sodium hydroxide and hydrogen in accordance with the formula Fe + 2NaCl + 2H2O FeCl2 + 
2NaOH + H2. This is of significance from the conservation aspect, as the iron chlorides are 
less soluble in water than sodium chloride but have a considerably lower melting point. 

The iron dissolved from the metal objects in the corrosion process has, to a large 
extent, been precipitated on material in the ambience because of the virtual absence of 
water flow. When, therefore, the Wasa was raised to the surface and the investigations 
started, the planking was resplendent with all manner of yellow, brown, black, and red 
shades. Stalactite deposits of up to half a metre in length hung from the ship's sides in 
the hold and on the orlop deck, usually down to a knee or strake of ceiling. The stalactite 
deposits have been analyzed in order to determine their composition and find out how they 
arose. 

The investigation showed that the stalactite formation consists of an inner core of 
magnetite, siderite, and pyrrhotite. The outer varicoloured fine powder appears to consist 
of oxidation products of these phases--iron sulphate, iron hydroxide, goethite and lepidocro- 
cite. The formations have an extremely light and porous structure and appear to be caused 
by the fact that the precipitation of iron took place on timber. This is confirmed by the 
presence of organic carbon. From the geological aspect the crystallization of magnetite is 
of interest through this 333-year-old "laboratory experiment". 

The investigations of the material reveal the following. The layer of mud on all decks 
had an estimated total weight of 700 tons. In appearance, it was partly black and partly 



159 



grey. The iron content in these layers of mud was 22 and 2 percent, respectively, or 
between approximately 14 and 144 tons. Furthermore, practically the entire hull was impreg- 
nated with iron, which resulted in the very much admired colour of black oak. The mean 
concentration of iron in the hull is 0.63 percent or about 3 tons of iron. 

In the drying out of the wooden sculptures, a deposit was formed on the surface. As 
there was doubt as to whether it consisted of a conserving agent or the like, or of precipi- 
tation of salts in the wood, an x-ray crystal lographic analysis was made. This showed that 
the white deposit consisted of mixtures of iron sulphates (FeS04-4H20 and 7H2O) and a greyish 
brown deposit which, after heating, had the diffraction pattern of pure iron oxide, Fe203. 

Analysis of textiles showed the iron concentration to be as much as 45 percent. The 
composition of precipitates on skeletal parts, sculptures, etc. after conservation and in 
the form of stalactites has been determined. Precipitates on skeletal parts consist of 
about 95 percent of an iron phosphate, vivianite, FeslPO^) •8H2O. Apart from iron, zinc was 
found in a concentration of about 1/20 of that of the iron, and also traces of manganese and 
phosphorus. Carbonate groups were also discovered, which renders it probable that the zinc 
is in the form of basic zinc carbonate. In the form of bivalent iron the vivianite is 
colourless, but is quickly oxidized in air to the trivalent iron phosphate without change of 
the cyrstal structure. In one case, a skeleton was found close to a gun carriage. Iron 
from rusted iron objects has reacted with the phosphate ions in the bone. The zinc probably 
derives from a bronze cannon. When the skeleton was exposed, the uncoloured white bivalent 
iron phosphate was converted into blue trivalent iron in half an hour. 

The reason for the continued corrosion of the iron objects after their salvaging from 
the sunken vessel has been explained earlier. I shall merely illustrate this condition by 
means of two practical examples. The speed of rusting varies. The disintegration can be 
delayed by means of various surface treatment methods. The cannon from the "Riksapplet" , 
which foundered in 1676, was salvaged in 1953. It was scraped, brushed, and treated with 
antirust oil and placed in the museum in 1954 (fig. 2a), After 18 years, the corrosion had 
proceeded so far that all ornamentation had disappeared (fig. 2b). The large iron pot from 
the Wasa was rusted to different degrees at the time of salvaging. Part had rusted all 
through, while other parts had their iron core intact. Attempts were made to rinse the 
parts with water, extract water with acetone, dry in vacuum, and vacuum impregnate with 
acrylic lacquer. The result was unsatisfactory, and continued corrosion was observed already 
within a few weeks. In the case of improper conservation or lack of conservation of the 24- 
pound cannon-balls from the Wasa, total disintegration may occur within a month. 

There has, therefore, been a great need to find a reliable method of conservation for 
iron. The desire has been for a method with which, after the treatment, one knows that all 
recesses, cavities, etc., which are inaccessible by conventional methods, are treated; also 
that one is not then forced to remove the rust but can instead preserve the surface which 
gives the object its correct form, and which often is ornamented. We chose reduction with 
hydrogen gas at raised temperatures. By means of this treatment, rusting is arrested com- 
pletely by eliminating water and oxygen in the material and, which is perhaps more important, 
all the chlorine. This is difficult to attain completely by other methods. 

The carbon content in the corroded part of the find varies very much depending on how 
much iron which has simply disappeared during the corrosion. Ten to 20 weight percent is 
very normal. Calculated on the reduced find, that is without oxygen, values up to 35 percent 
of carbon might be obtained. This means that the carbon content is far higher than in the 
original find. When choosing the reduction temperature, it is desirable that the carbon 
content should be retained intact. We recommend normally a reduction temperature of between 
600 and 700 °C, and the closer to 600 °C the better. These temperatures related to the 
reduction of objects solely of iron, and give the best results for this purpose. The decar- 
bonization at 600 °C is negligible. If the iron object contains other metalSj one may be 
forced to reduce at a lower temperature, e.g._, 300-400 °C, which takes a longer time. At 
higher reduction temperatures, 800-1000 °C, the speed of reduction is higher but leads to 
decarbonization in the rust layer of the find. Figure 3 shows the hydrogen furnace used at 
the Wasa laboratory. 

For the reduction it is best to use 100 percent dry hydrogen gas or a mixture of hydro- 
gen and nitrogen. With the pure gas, reduction time is shorter. An important point to be 
stressed is that the hydrogen must be dried from water before entering the reduction vessel 



160 



Figure 2a. Cannon from the Riksapplet, 
foundered 1676, salvaged 1953. As the 
cannon appeared, after soaking in water 
and boiling in paraffin, when placed on 
view in 1954. 



Figure 2b. The same cannon after 18 years 
on exhibition. Expansion caused by the 
continued production of rust has caused 
the surface to crack and the ornamenta- 
tion to drop away. 



161 




162 



to avoid decarbonization. As no drying is done of the find before the reduction process, 
the heating period to get rid of the free water must go very slowly to avoid cracking when 
the water evaporates. Also, the cooling time after reduction must be rather long. To be 
on the safe side, cooling in the furnace should be allowed to go to room temperature as, 
in some cases, porous superficial layers may have converted into a more or less pyrophoric 
iron which on admission of air may start to oxidize and form oxides once again. A good help 
in keeping a check on the reduction process is if one can check for water in the exhaust gas 
or arrange for suspension of the find in a scale during the reduction. A normal schedule 
for, for example, cannon-balls of 24-pound weight and with a total weight of charge of up to 
60 kg can be run in a week as follows: 



Operation Time (hours) 

Heating to 200 °C 8 

Maintenance at 200 °C for '^^16 

Heating to 400 °C 8 

Maintenance at 400 °C for ^^16 

Heating to 600 °C 5 

Maintenance at 600 °C 18 
Heating at 650 °C 5 
Cooling to room temperature 48 hours 



To prevent the rusting beginning again, the reduced objects were vacuum-treated with paraffin. 
The antirust agent used was 2 percent VPI (Shell patent). Accelerated rusting tests in a 
humidity chamber with admission of air during 10 months showed that the object tested had 
acquired a fully satisfactory protection against corrosion. The aim of the vacuum treatment 
of the reduced objects with molten paraffin is, as far as possible, to fully fill the pores 
in the entire object. Antirust agents with solvent can also be used. The degree of filling 
for the antirust agent in question is proportional to the concentration in the solution. 
This implies that one attains a degree of filling of 25-35 percent. This method is used 
when the requirements of increased mechanical strength of the objects are not unduly strict. 

Though the furnace was specifically designed for Wasa finds, it has since come into use 
for the conservation of other valuable finds. Among these is a viking sword (fig. 4). After 
the hydrogen reduction of the sword, brass threads decorating the hilt can clearly be seen. 

Hydrogen reduction is not an entirely unhazardous procedure and a few words should be 
said about the use of hydrogen as a reducing agent. In the first place, it must be remem- 
bered that hydrogen is very liable to unite with oxygen of the air. This reaction, if it is 
at high temperature, is very violent and is referred to as an oxy-hydrogen explosion. Very 
careful precautions must therefore be taken when working with hydrogen. For work with gases 
which may explode, there are certain relations, among which the flammability range is one of 
the most important. The concentration range within which not only combustion but also 
explosion is possible is called the flammability range and its limits, the lower and upper 
limits of flammability. The limits for gases are usually stated in percentage by volume and 
for hydrogen the limits are 4 to 75 percent. The mixture of hydrogen with air is thus 
nonflammable so long as the content of hydrogen exceeds 75 percent; between 4 and 75 percent, 
the mixture is flammable and explosive; below 4 percent it is again nonflammable. For the 
inflammable mixture to ignite or possibly explode, however, the gas mixture must be heated 
I to, or come into contact with, substances which have attained the minimum spontaneous igni- 
j tion or auto-ignition temperature of the gas mixture, which in the case of hydrogen and air 
is around 510 °C. If such an inflammable gas mixture is ignited inside a closed chamber, 
an explosion will take place. If, on the other hand, the gas mixture is allowed to flow 
freely out of the burner (as for example, a bunsen burner) a very high speed reaction 
admittedly takes place between the gases but it is not explosive. 

If, however, one always carefully drives off all of the oxygen in the air from, or to 
express it more simply, ventilates the reduction vessel before the reduction, one can then 
without risk admit the hydrogen into the red hot vessel. Ventilation is done most simply by 

j flushing out the vessel with an inert gas such as nitrogen during the time of heating to the 

;| desired temperature. 



163 



Figure 4a. Hilt of a viking sword before hydrogen reduction 




Figure 4c. The viking sword after hydrogen reduction. 




Figure 4d. The hilt of the viking sword after treatment, showing 
the decoration of brass threads. 



Problems And Ethics In The Conservation Of Iron Objects 

The question is, will there be any changes by using the reduction method in the hardening 
of the find and will there be any other metal lographic changes of significance or changes in 
volume. 

On heating to 600 °C, hardening disappears almost entirely. One should, however, keep 
in mind that lower- temperature heating also has an effect on hardening, for example by hot 
water extraction or wax treatment. Hardening disappears gradually also with time. It must 
therefore be understood that the degree of hardening possessed today by a find deriving, 
say from the 15th century, is not the same as its original degree of hardening. It is not 
possible to extrapolate the initial hardness from the present hardness. A sample taken 
before any preservation begins is to be recommended if the degree at present of the hardening 
is to be recorded. 

If a metallurgical examination is to be made, a sample must be taken, which means some 
destruction of a part of the find. This sample should be taken as soon as possible after the 
excavation and before preservation. 



165 



other metal! ographic changes, such as the production of austenite, do not happen when 
using a reduction temperature of less than 600 °C. 

In work on conservation, one is interested in restoring the details to their original 
form. By means of hydrogen gas reduction the volume of the find can be kept the same as when 
it was discovered. 

Concerning the colour, it will, of course, change from the brown rusty colour the find 
has when it is found, to black, as the colour the find had when it was in use. 

It is, in my mind, of the greatest importance for purposes of exhibition and pedagogy to 
show an intact find. It is obviously unsatisfactory to show a collection of rusty iron 
objects, giving the viewer the wrong idea of what they once looked like and the impression 
that our forefathers used tarnished weapons. 

In conclusion, it may be said that the experience of reduction of the iron objects at 
National Maritime Museum - Wasa Shipyard during the twelve years in which this work has been 
going on is extremely satisfactory. Owing to the great reliability of this conservation 
method, no new rusting, or consequent cracking, has arisen. Purely aesthetically, and from 
the museum aspect, the objects have an attractive and correct appearance, i.e., they look as 
they did in use. 



Discussion 

M. Pourbaix: You have made some comparisons between the hydrogen reduction process and 
some processes with aqueous solutions. Are there cases where processes with aqueous 
solutions might be preferred? May aqueous processes lead to a complete removal of chloride? 

L. Barkman: We made comparisons with the various aqueous solution methods, but we did not 
succeed in removing the iron chlorides. That was a poor conservation method. 

E. Esaalante: Do you clean and recycle the hydrogen or do you just vent it? 

L. Barkman: We do not recycle it. We use such a slow stream of hydrogen that we simply 
ignite it after it has passed through the reduction chamber. 

E. Esaalante: In the film it was mentioned that the water has to be replaced as the wood 
dries. What is it replaced with? 

L. Barkman: Polyethylene glycol. 

J. Kruger: Have you looked at the metal 1 ographic structure of the reduced cannon balls as 
compared to the metallography of cast iron that has not gone through this corrosion and 
then reduction? 

L. Barkman: Yes, this was done at the Royal Technical Institute in Stockholm, but I do 
not have that report with me so I can not respond to that question. 

D. Fiechota: On hydrogen reduction, how long is the treatment from the first heating of 
the specimen until it is removed from the oven? 

L. Barkman: We normally run one treatment a week. 

E. Bump: What is the explosive power of one cubic foot of hydrogen? 
L. Barkman: It will blow away the furnace. (Audience laughter). 



166 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburq 
Maryland, March 17 and 18, 1976. Issued July 1977. 



RESTORATION OF LARGE GILDED STATUES USING VARIOUS ELECTROCHEMICAL AND 
METALLURGICAL TECHNIQUES 



Fielding Ogburn, Elio Passaglia, Harry C. Burnett, 
Jerome Kruger, and Marion L. Picklesimer 

National Bureau of Standards 
Institute for Materials Research 
Washington, D.C. 



Four gilded bronze equestrian statues, erected in 1951 in Washington, D.C, 
had, by 1968, shown severe signs of deterioration. Extensive surface pitting 
and discoloration were observed, steel structural members had disintegrated, 
severe cracks were evident, and numerous holes were clearly visible from the 
interior. Restoration included replacing steel members with brass, filling 
cracks and pits with tin-silver solder, refinishing with gold by electroplating, 
and sealing the surfaces with an acrylic lacquer (Incralac). 

Key Words: Bronzes; deterioration; gilding; metal finishing; refinishing; 
restoration; statuary. 



1. Introduction 

In Washington, D.C, on the Memorial Bridge Plaza between the Lincoln Memorial and 
the Potomac River are four large gilded bronze equestrian statues. The statues, which are 
mounted on stone plinths, are about 6 m long, 6 m tall, and 3 m wide. They were cast in 
bronze and then gilded by applying a layer of gold amalgam and heating to evaporate the 
mercury so as to leave a thin layer of adherent gold. 

These statues arrived from Europe and were set in place in 1951. They stand above 
the tidal waters of the Potomac River near two streets carrying very heavy automobile 
traffic and are subject to about 100 cm of rain each year. In 1971, after some twenty 
years of exposure to the environment, the condition of the statues had deteriorated to the 
point where the structural condition and the surface finish both required that action be 
taken to restore and preserve these works of art. 

The four equestrian statues are the responsibility of the National Capital Parks of 
the Department of the Interior. This organization has had a great deal of experience with 
the maintenance of roads, parks, public monuments, and statuary. That experience, however, 
did not include the repair and restoration of massive gilded bronze statuary and it looked 
elsewhere for advice and guidance. Experts in the fields of mechanical metallurgy, corro- 
sion, electrodeposition, and organic coatings from the National Bureau of Standards were 
made available to the National Capital Parks and, within the areas of their expertise, 
provided advice and guidance. 

This paper summarizes the observations of the NBS team. The restoration was carried 
out by the National Capital Parks. The various materials and procedures used were selected 
by the National Capital Parks on the basis of artistic, economic, and political consider- 
ations, as well as the technical advice of the National Bureau of Standards, the Department 
of the Navy, and the International Copper Research Association. 



167 



2. Condition of Statues 



A. General Exterior Appearance 

The names of the four statues, from north to south, are Aspiration and Literature, 
Music and Harvest, Sacrifice, and Valor. The last two. Sacrifice and Valor, looked very 
much alike. The upper surfaces, washed by rain, were heavily encrusted with corrosion 
products and dirt, dark green to black in color. The vertical and near vertical surfaces 
were streaked by the runoff of the rain water. 




The two northerly statues, by comparison, looked much worse and Music and Harvest was 
obviously in poorer condition than Aspiration and Literature. Figures 1 and 2 show the 
condition of these two in 1968. Only the upward facing surfaces were washed clean by rain; 
all the other surfaces were dirty with corrosion products so that the impression was of 
black surfaces with some golden highlights. 

B. Condition of Exterior Surfaces 

Upon the removal of the dirt and corrosion products, the underlying conditions of the 
gilded surfaces became evident. No one condition or set of conditions prevailed. Some 
areas had no objectionable defects and the gold layer was sound and intact and adherent. In 



168 



some small areas, gold was not present, but this seems to have been of relatively rare 
occurrence. More frequently, the gold layer was perforated. That is, there were many 
i breaks in the gold layer. Also, a significant portion of the gold was poorly adherent and 
could be easily removed by wire or nylon brushes. 

The most impressive condition was the very large number of small depressions and pits. 
Their appearance strongly suggested that most were present in the original castings, some 
being shrinkage pits and some being the remains of gas bubbles. They were present in many 
^, sizes up to several millimeters in diameter and in many shapes and depths. Many extended 

all the way through the castings. We presume that many of these had been enlarged by corro- 
, sion and some may have been created by corrosion. We would expect that galvanic corrosion 
j would be likely wherever there was a break in the gold. Most of our observations of these 
! pits were made after the gold had been removed by sandblasting. Many were evident, however, 
I where the gold had not been removed, but we presume that the gold layer did cover up some of 
' these defects. 

Figures 3, 4, and 5 are micrographs of cross-sections of corings taken from the statues. 
They show the gold layers, some intact, some with breaks, and some looking spongy. The dark 
layer between the gold and the bronze was always present in the six different cross-sections 
examined. An electron microprobe analysis shows it to consist of the same metallic elements 
(Cu, Sn, Zn, and Pb) in about the same proportion as in the bronze, but with oxygen added. 
The surface of another specimen was examined by x-ray fluorescence after some loosely ad- 
hering gold was lifted off. Sulfur and arsenic were found to be present. The sulfur could 
have come from the atmosphere or from a sulfuric acid treatment reported to have been given 
I the bronze just prior to fire gilding. 

The micrograph in figure 4 shows porosity within the cast bronze. For the most part 
this is of no particular significance except for the exterior surface condition described 
earl ier. 

The surface of one specimen from one of the statues was analyzed for residual mercury 
and 0.6 mg/cm^ of mercury was found, but we have no assurance that this is representative of 
I all surfaces. 

X-ray diffraction patterns of the green black materials that had formed over the gold 
surfaces demonstrated the presence of CaS0i+-2H20 (gypsum) and CuS0i+/3Cu(0H)2 (brochantite) . 
Brochantite is the major constituent of natural patina. The only probable source of the 
gypsum would be the mold materials used to cast the bronze. There was considerable gypsum 




Figure 3. Micrograph of cross-section of piece removed from Aspiration and 
Literature in 1968 showing spongy gold deposit on surface of bronze casting. 



169 



Figure 4. Micrograph of cross-section of plug removed from Sacrifice in 1968 
showing broken gold layer over porous bronze casting. X240. 




Figure 5. Micrograph of cross-section of piece removed from Aspiration 
and Literature in 1968 showing gold coating over bronze casting. 

inside the statue from which an analyzed sample was taken and it is reasonable to expect 
that the gypsum was deposited on the outside from water seeping from the inside through 
cracks and pores. 



C. Structural Conditions of Bronze 

Several cracks in the bronze shell of Music and Harvest, and of Aspiration and Litera- 
ture were evident (figs. 6 and 7). Generally, there was no gold within the cracks suggesting 
that many of them either formed or grew after the gold had been applied. These cracks may 
have contributed to the exterior corrosion, but only two of them appeared to be structurally 
significant. One crack was about 2 m long, extending more than halfway around one way of 



170 



Figure 6. 



Crack in Music and Harvest. 



Figure 7. Crack in Music and Harvest. 



Music and Harvest. Another was extended most of the way around a feather on the lower 
right wing of Aspiration and Literature. These two cracks were potential sites of structur- 
al failure. 



D. Condition of Interiors 

Three of the statues could be entered through the supporting plinths with the aid of a 
ladder or scaffolding. The fourth statue, Valor, was entered through a hole cut in the back 
of the horse. It is, however, open to the hollow plinth through the legs. 

Upon examining the statue from the inside, with no light in the interior, one became 
immediately aware of numerous holes letting in pinpoints of light. Some of these were 
associated with cracks, seams, and welds. Some were simple pores extending through the 
casting. 

The interior surfaces seemed perpetually damp or even wet. Any blind cavities or 
recesses that could hold water were filled with water. Some of this water may have come 
through holes from the exterior, but it seems more probable that most came from condensa- 
tion. The situation in these statues is very favorable to condensation because of the large 
temperature changes that the bronze would undergo almost daily, the rapid heat transfer 
characteristics of bronze, and the ready source of moist air from the Potomac River through 
extensive chambers under the bridges which connect through the plinths to the interiors of 
the statues. At night, condensation over the entire interior surfaces can be expected and 
air circulation would bring in moist air from under the bridges. Much of the condensate 
could easily run down and collect in the recesses. During the day, evaporation of that 
water would be restricted if for no other reason than that air circulation would be limited. 
The air would be heated from the top and the hot air would be trapped in the essentially 
closed system. This air, of course, would contain considerable water. 

The interior surfaces of Aspiration and Literature and of Music and Harvest were 
covered with corrosion products and residual mold materials. The latter is presumed to be 
mostly gypsum, possibly mixed with sand (fig. 8). It was present in considerable quantity, 
expecially in the wings of the horses, and may have played a part in holding water inside 
the two statues. Sacrifice and Valor were relatively free of the molding materials and a 
green patina was present over most of the surfaces. 

The statues were cast in a number of pieces that were subsequently fastened together. 
The two southerly statues. Sacrifice and Valor, were welded together and the other two were 
bolted together at flanges integrally cast on each piece. Music and Harvest also has 
several pieces held together along a centrally located seam. This seam is held together by 



171 




"tie-plates" bolted to the pieces being joined. The bolts, nuts, studs, washers, and 
"tie-plates" throughout the two statues were predominantly steel with some being of brass or 
bronze. These steel members, in contact with the bronze in a wet environment, undoubtedly 
formed an active galvanic couple. As would be expected, the steel was badly corroded and 
many of the parts no longer served a useful purpose (figs. 9 and 10). 





Figure 9. Collection of corroded 
pieces removed from Music and 
Harvest. 



Figure 10. "Corrosion Rose." Remains of corroded 
steel bolt head. 



The statues contained steel tie rods or braces and each was anchored to its plinth with 
a steel framework. These steel members had been coated with a bituminous material, but had 
nevertheless corroded (figs. 11 and 12). In some places the coating had come off in local 
areas. In other places the corrosion had taken place under the coating. Some of these 
structures were still sound, but others were obviously not. One tie-rod had corroded all 
the way through. 



172 




3. The Restoration 

The restoration of Aspiration and Literature and of Music and Harvest was carried out 
during the fall of 1971 and that winter. The restoration of Sacrifice and Valor was started 
in November of 1972 and completed that winter. 



A. Interior Repairs 

Mold material was removed to the extent practical with reasonable effort. This was 
done principally to facilitate the other work to be done. It was hoped, however, that its 
removal might reduce the corrosi veness of the inside environment by reducing the surface 
areas for condensation and water absorption, and might reduce the formation of gypsum 
deposits on the outside surfaces. 

All the steel fasteners, braces, tie rods, anchoring structures, etc., were replaced 
with naval brass. This should provide a more durable structure. Some 1000 bolts were 
replaced in Music and Harvest and Aspiration and Literature, the two northerly statues. 



173 



B. Repairs to the Bronze Shells 

The bulk of the dirt and corrosion products on the exterior surface was removed by 
treatment with steam and a detergent. The remaining corrosion products and the loose gold 
were removed with motor driven rotary wire wheels. The more adherent gold was removed by 
sand blasting with No. 1 grit sand. 

It had been planned to fill all cracks by welding and/or brazing. It was found, 
however, that only a few of the cracks could be welded, as it was not possible to preheat 
the metal at many of the cracks to a suitable temperature for welding. These latter cracks 
were filled with a solder, 97 percent tin - 3 percent silver (fig. 13), which was used 
instead of lead solder because it is stronger and more corrosion resistant. The large crack 
on Music and Harvest was tied together by bolting plates across the crack on the inside 
(fig. 14). The threaded studs penetrating the shell were ground and smoothed to the contour 
of the outside surface. 




Figure 14. Inside view of crack held 
together by brass tie plates and of 
one end of a brass tie rod. 



Many pits were routed out and filled with the same solder. There were, however, far 
too many of the small pits to make this a feasible method of filling all of them. A conducti 
epoxy was used to fill some of the pits, but because it was difficult to electroplate over 
that surface, most of the pits were left unfilled. 



C. Surface Finishing 

The exterior surfaces were refinished by electroplating with nickel and then with gold. 
This was accomplished by the brush plating technique which has found considerable use for 
repairing plated surfaces of machinery and aircraft equipment. It has been used for gold 
plating new church domes, but we are not aware of its having been used to gold plate statuary 



174 



ij and large bronze castings. For this application the process is unique. In brief, a carbon 

I rod is wrapped with absorbent cotton and cotton gauze and connected to the positive pole of 

1 a direct current power source. The negative lead is grounded to the statue. The cotton 

covered rod is dipped in the plating solution and then swabbed back and forth over the 

, surface to be plated (fig. 15). As the current flows from the rod through the solution to 

I the bronze, the nickel or gold is deposited as a smooth adherent layer. The actual process 

I is, however, more sophisticated than indicated here. 




Figure 15. Nickel Plating. 



For this application the nickel and gold were each applied in two or more layers and 
each layer was buffed with a greaseless abrasive pad. The purpose of the buffing was to 
minimize the porosity of the coating and to provide a surface with an appropriate appearance. 

For the nickel plating, the following sequence of operations was typical. This was 
carried out over small areas at a time. The area might be anywhere between a few square 
centimeters and about a square meter, depending on the shape and accessibility of the 
surface and the skill of the plater. 

1. Mechanically clean with a rotary 
wire brush and/or a rotary abra- 
sive flap brush. 

2. Cathodically clean with a propri- 
etary alkaline electrolytic clean 
ing solution 

3. Water rinse. 

4. Nickel plate. 

The solder and some areas that were particularly difficult to plate were first plated 
with about 10 ym of copper. 

A typical sequence for the gold plating was as follows: 



5. Water rinse. 

6. Buff and activate with an abra- 
sive pad. 

7. Nickel plate. 

8. Water rinse. 



175 



1. Buff and activate the nickel 
with an abrasive pad. 

2. Clean cathodically with an 
alkaline cleaner. 

3. Water rinse. 

4. Gold plate. 

5. Water rinse. 



6. Brunish with an abrasive pad 
or levigated alumina. 

7. Water rinse. 

8. Repeat steps 4 to 7 two more 
times. 

9. Air dry. 

10. Apply acrylic lacquer (Incralac) 



These finishing cycles were not always followed, but varied from place to place, 
depending on the surface conditions, surface contours, accessibility, and operator technique 

By the nature of the process and individual variability, the thickness of the metal 
coatings can be presumed to have varied widely from place to place. Also, there was a 
deliberate effort to plate more heavily over the pitted areas than over the smooth surfaces. 
The thickness averaged about 12 ym of nickel and 4 ym of gold, as judged by the quantity of 
plating solutions used. 



A clear acrylic lacquer containing benzotriazole was applied over the gold, at least 
two coats by brush and two coats by spraying. This lacquer, "Incralac" was developed by the 
International Copper Research Association for use on bronze. It is corrosion inhibiting, is 
not degraded by ultraviolet light, does not discolor, and does not flake off. 

It has been anticipated that weathering of the restored statues would develop local 
corrosion sites that were not, for one or another reason, adequately refinished during the 
initial restoration. Consequently, Aspiration and Literature along with Music and Harvest 
were closely examined in the fall, some 10 months after completion of the initial refinishin 
Numerous small holes, cracks, and seams were sites of bronze corrosion products and/or of 
white deposits which, presumably, were gypsum. Water saturated with gypsum seeped through 
pores from the interior to the outside and evaporated, leaving a white powder. These 
defects were not serious, but did require repair. 

The smaller defects were cleaned off and lacquered. The larger ones were routed out, 
filled with solder, replated, and lacquered. 

This retouching was repeated two years later, but the number of sites involved was very 
much less and were too small to be seen from the street. 



4. Renewal of Lacquer 

Within a few months after the initial refinishing of the first two statues, there was 
a distinct darkening of extensive areas of the gilded surfaces. The discoloration was very 
irregular and uneven, and it continued to develop for awhile and then stabilized. A close 
examination revealed that some dark materials had formed at the gold-lacquer interface. 
When the lacquer was removed with a solvent, this dark material came off with the lacquer. 
The darkening did not recur after relacquering the clean surfaces. 

After making several tests, it was concluded that the darkening was associated with 
residue from spent gold plating solution; that the gold plate had not been adequately 
rinsed after plating and before lacquering. With time the residue underwent a change and 
became dark and opaque. 

This conclusion was confirmed by preparing laboratory specimens with various amounts of 
residue left on them, lacquering and exposing. Darkening of the specimens with spent 
solution was obtained, but not on those that had been thoroughly cleaned. 

In 1974, all the darkened areas were cleaned off and relacquered. Since then there 
has been no evidence of further difficulty of this nature. 



176 



5. An Evaluation of the Restoration 
gloss imparted by the llcquer (fig °6) Wilh thS L 't^^ ^^^^^ 




Figure 16. Aspiration and Literature 
after restoration. Winter, 1972. 



Structurally, the restored statues should be sounder. There is some concern that some 
detrimental corrosion on interior surfaces may occur where the tin-silver solder comes 
through to the interior of the statues. The bimetallic couple in the wet environment may 
have adverse effects, but they should not be very serious. 

The crucial aspect which requires observing closely in the future is the behavior of 
the gilded surface. There is, undoubtedly, some porosity in the gold coating, especially is 
those areas which have less than the average thickness of gold. More significant, however, 
is that there are a large number of pits which could not be gold plated. That is, the gold 
would not completely plate over the inside surfaces of the pits (fig. 17). These spots or 




Figure 17. Pitted surface after 
gold plating. 



177 



pores where the bronze is exposed to the atmosphere at points of contact with the gold are 
potential sites for severe galvanic corrosion. The acrylic lacquer (Incralac), of course, 
will inhibit such corrosion, but only as long as it is there. It is expected to be worn off 
by the elements in about five years and there is always the possibility that it will not be 
reapplied when it should. Fortunately, the pits are pretty well filled with the lacquer so 
that the greatest thickness of the lacquer is where it is needed most. Also, we anticipate 
that the erosion of the lacquer in the pits will be much slower than over the general surface 

With periodic inspection, cleaning, repair of individual flaws as they develop, and 
periodic lacquering every five to ten years, the statues should keep virtually the same 
appearance indefinitely without expensive repairs. 



Discussion 

F. Brown: What is the mechanism of the cracking in those horses? 

J. Xruger: Since the cracks occurred at areas adjacent to where the sections were bolted 
together and under some stress coupled with presence of ammonia like species in the en- 
vironment, we felt that some of it may have been stress corrosion cracking. Of course with 
bronzes it is not as likely as with brass but I think that there was some stress corrosion 
cracking, 

M. Pourbaix: I cannot speak about this specific problem as I am not familiar with the 
details. I can say, however, that the problem of pollution exists everywhere. We all know 
of the problems now existing in Italy and in France. It is also true that when you have the 
combination of a polluted atmosphere and a porous gold coating, the structure will be 
destroyed due to the high electropotential . This may be avoided if a completely non-porous 
gold coating is applied. 

I also wish to mention that in some countries such as the German Democratic Republic, 
the corrosion aspects of a structure must be considered in the planning and design stage. 
I do not wish to propose that this is an example that we should follow, however. Professor 
Sweible at the Center of Corrosion in Dresden, has a staff of 200 corrosion engineers. 
Anyone who is building a metallic structure in Eastern Germany must ask the advice of this 
organization and apply the specification recommended. We all know that 80 to 90 percent of 
all corrosion damage may be prevented if the proper steps are taken in the beginning. I 
might also add that Professor Sweible will be coming to Brussels next April on the occasion 
of our 25th Anniversary. 

The horses are wonderful structures and perhaps the procedure that was used is the 
best, provided that the brush system leads to a non-porous coating and that the Incralac 
is permanent. 



178 



National Bureau of Standards Special Publication 479. Proceedings of a Seminar, 
Corrosion and Metal Artifacts--A Dialogue Between Conservators and Archaeologists 
and. Corrosion Scientists held at the National Bureau of Standards, Gaithersburg, 
Maryland, March 17 and 18, 1976 Issued July 1977. 



PROBLEMS OF RETRIEVAL AND RETENTION OF ARTIFACTS IN FIELD EXCAVATIONS 



W. Trousdale 

The Smithsonian Institution 
Washington, D.C. 20560 



I feel obliged to make some preliminary remarks, not by way of apology, but as preface. 
I must state at the outset that the corrosion of metal artifacts does not absorb the prime 
time on any archeological excavation of which I am aware. It can, however, be a matter of 
greater or lesser importance with respect to the individual dig, and these circumstances are 
highly variable. I am informed, for instance, that an Arctic archeologist may have a 
serious problem with the disintegration of metal artifacts removed from a zone of permafrost. 
That is not a problem I have ever faced in the deserts of Syria or Afghanistan during the 
last twelve years. 

It is quite impossible for any two archeological excavations to be as alike in dealing 
with investigative problems as two laboratories in one country, or even in separate countries. 
While laboratory conditions can be duplicated exactly, or precise equivalent tests conducted 
concurrently in the same, or separate laboratories, it remains as true today as ever that 
all archeology is a process more akin to destruction than to conservation, in spite of the 
virtuous and ecologically conscious feeling which may be derived from mandatory back-filling 
in some countries today. It is by no means clear that all excavations have as their ulti- 
mate goal the elucidation of some direct or correlative aspect of human presence on earth. 
It may be no more than a personal prejudice that believes they should have such a guiding 
principle. Beyond this, every excavation is highly individual, governed by geography, 
climate, politics, local economic imperatives, varying cultural pressures, ethnic, age, 
cultural, and intellectural diversity of staff, all existing as probable greater determinants 
in a field excavation than in the more readily achieved ideal laboratory. Except for the 
fact that any excavation normally entails digging, there may be little or no common ground, 
a point which for the last hundred years and more has been forcefully exhibited in the way 
in which archeologists speak of each other's work. It is, perhaps, to this well-known 
circumstance I allude in stressing that what follows here is a personal view. I sail upon 
this sea of corrosional erudition under the much more pal id colors of the general ist, which 
is what the practice of archeology in the field, irrespective of one's normal penchant, 
requires one be. Perhaps my remarks may be looked upon as a not altogether irrelevant 
diversion from the sobriety of this occasion. 

In 1815, the Honorable Mountstuart Elphinstone, who was not able to visit Sistan, 
enthused: "There is no country to which an admirer of Persian poetry and romance will turn 
with more interest than to Seestaun, and there is none where his expectations will meet with 
so melancholy a disappointment." Nearly a century later. Lord Curzon, who chose not to 
visit Sistan, wrote: "Seistan is one of the most unattractive, the most inhospitable, the 
most odious of places in the world. It is a country of marshes and of swamps, of sands and 
solitudes, of extreme heat and extreme cold, famous for a wind the most vile and abominable 
in the universe, presenting at all seasons of the year dangers to life which can scarcely be 
realized by those who only read of them at a distance." 

In February 1971, I signed, on behalf of, I believe, the Smithsonian Institution, a 
contract with the late Royal Government of Afghanistan to undertake the archeological ex- 
ploration and excavation of the southwestern portion of the country (fig. 1). This was not 
quite as mad an act as it at first seems, since it was assumed the inhabited portion of this 
region would be defined by the Helmand River Valley, much as the Nile Valley contains 
virtually all the vestiges of ancient Egyptian civilization. In point of fact, I was wrong 
in this belief. Irrigation canals 100 miles in length, a few perhaps longer, and utilization 
of run-off from high mountains to the south in Pakistan, meant that people in very early 
times lived at great distances from the river. 



179 



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Figure 1. Map showing southwestern portion of Afghanistan. 

From the very beginning, we felt very much as we imagined archeological explorers felt 
entering the Nile Valley, or the Tigris-Euphrates region, early in the 19th century. The 
comparison may be easily extended. For our work, we had no model; for our project, no 
paradigm; and for our scope, no parameter. (These are loan words from the New Archeology.) 
This is at once the project's limitations and weakness, and, we hope, its breadth and 
value. We worked within a time frame of unknown termini. No written sources, copious as 
they may be, had any immediate relevance, and in the final analysis, no significant perti- 
nence to anything we found. This has, in the end, resolved itself into a matter with which 
others who may follow can cope; we have no explanation for it. We liked to remark, ironi- 
cally, sitting around the campfire, that our labors were conducted with all the advantages 
of a 19th century exploratory mission to Inner Asia. But that is not the truth. Those 
missions were, in the main, far better outfitted and financed. For the thousands of exposed 
burials covering the playas and spoil banks of great and small canals, we had no physical 
anthropologist; for the rich and puzzlingly varied, curiously clinging, flora and fauna of 
the basin, we had no botanist or zoologist. For the life of the hamun, or land-locked fresh 
water lakes, and the Helmand River, no limnologist. With the more arcane subjects of the 
modern day paleobotany, sedimentology , etc. --we could do nothing. Certainly we had no 
conservator, and if put to it, I believe a corrosion scientist would have been pretty far 
down on our want 1 ist. 

Because of an enlightened policy on the part of at least a few individuals, first at 
the Johns Hopkins University, and afterward at the U.S. Geological Survey, we have had from 
the beginning an environmental geologist. The amazing success of his work on the formation 
of dunes and transport of sand by water and wind, of his studies on deflation and aggradation, 
does indeed cause me to lament that we had with us specialists in so few disciplines. But 
there was virtually nothing to be done about this. 

After more than a year spent in the field thus far, in an area of forty thousand 
square miles, without a town, without a road, without telephones, electricity, food, or 
potable water, we know at least something of how much we do not know, which we think is an 
appropriate first step. Our work is far from finished. Archeologically, we have surveyed 
in some detail more than one hundred and fifty sites and excavated wholly, or in part, 
seventeen. This is no more than a non-random sampling. The time range ends certainly 
around 1500 A.D. It may begin in the 4th or 5th millennium B.C. 



180 



Our tasks are similar in that the problem of confronting continuously altering materials 
intelligently is always with us. Our tasks are separate in that for the conservator, the 
problem ts the profession, whereas for the field archeologist, circumvention of the problem 
is the expedient. The conservator devotes as much time to an object as it may require--ten 
minutes, ten days, ten weeks. In the field, the archeologist has to deal daily with perhaps 
a thousand objects, very few of which present exactly the same problem, and have to be 
studied minimally to determine even this basic bit of data. I no longer wonder why final 
reports of excavations are often so late in coming out; I marvel that they ever appear. 




Figure 2. The Shahr-i Gholghola site. 

The foremost problems in the field are those of retrieval. Without the object, all 
the rest is moot. The major component of this aspect of our work is simply logistics. Ever 
mindful of the dire forebodings of Lord Curzon, when we go out into the desert we bring with 
us everything we need for a minimum of ten weeks: all the food, all the water, all the 
gasoline. The country through which we must travel is frequently rough and extreme desert, 
and it takes two long days of driving to negotiate the 275 miles to our base camp. Some of 
the sites in the region, such as Shahr-i Gholghola (fig. 2) are quite visible, and though 
they present insurmountable problems of sand removal, much can be learned from surface 
indications. It is impossible to move this sand at a rate faster than it accumulates. 
Because of the high winds that exist in the area, we have to grab at sites when they appear 
from under sand dunes. That is, we have to be ready to deflect our attention, if possible, 
onto a site such as the 2000-year-old sanctuary complex in figure 3 when it is available. 
We did clear the major portion of this in 1973, and when we returned the following year it 
was largely covered by fresh sand. We have to complete every year what we start because 
most excavated areas cannot remain open from one year to the next. Smaller sites may be 
completely sand-buried during a given field season. Certain other sites are so deeply 
covered by permanent sand dunes, owing to the aerodynamics of the area, that we cannot 
contemplate excavation of them. The most dramatic indication of the wind and sand problem 
is represented by a small group of mausolea in an Islamic period cemetery (fig. 4). In 
1971, when we first saw them, they were sand free. When we returned in 1972 the cemetery 
was almost completely concealed. The following year, the area was again emerging. For over 
about five hundred square miles of the Sistan Basin, it is almost impossible to put a foot 
down without stepping upon an artifact of some sort. Ours is a problem of selection, not of 
paucity. This concentration of surface sherds is caused by deflation in the area. In other 



181 




Figure 3. A 2000-year-old sanctuary complex. 




a 

Figure 4a. Small group of mausolea in an Islamic period cemetery. 
View taken in 1971, shows them sand free. 



182 




b 

Figure 4b. Small group of mausolea in an Islamic period cemetery. 
View taken in 1972, shows them almost completely covered with sand. 




c 

Figure 4c. Small group of mausolea in an Islamic period cemetery. 
View taken in 1974, show the mausolea again emerging. 



183 



Figure 5. Illustration of destruction of ceramic artifact by the abrasive action 
of wind-driven sand. 



parts of the region we find aggradation proceeding at a faster rate than deflation, and the 
concentration of potsherds exists at varying depths below the present land surface. 

In our excavations, most of our retrieval problems are related to ceramics rather than 
to metal. When these are exposed on the ground surface for long periods, they may be 
almost entirely destroyed by the abrasive action of wind-driven sand (fig. 5). The rate of 
deterioration of the ceramic, or its response to the abrasive action of the wind, depends 
directly on the hardness of the ceramic in the first place, and we have found that some of 
the latest ceramic in our area deteriorates much more rapidly than other much older wares. 

But even if we do get the object out of the ground, what happens to it then? What are 
the problems of recording and retention? One of our basic problems of recording can be 
summed up by figure 6, a notebook with very acid paper which is not going to last very 
long. With a stationery budget of 20 dollars, we are forced to rely upon the locally 
available paper and that happens to be such Chinese student notebooks. Much more expensive, 
but equally unsatisfactory, were display albums with plastic envelopes (fig. 7). They are 
electrostatic, so that in a very short while they are completely covered with the blowing 
silt; and worse still, in the heat of the area they begin to melt and the paper sticks to 
them. It is especially bad in the case of the xeroxed materials. When you remove the 
Xerox, the ink stays behind on the plastic, and when you put it back in you have to line it 
up exactly, otherwise you have two images. 

Closely allied with the problems of retrieval and retention is that of transport. For 
some expeditions, that may not be as great a problem as it is on ours. Every object we find 
is at least 600 miles from the storerooms of the Institute of Archaeology in Kabul, which 
constitute a wholly separate problem. The first 275 miles, or so, of this distance is over 
extremely rough terrain. We cannot hope to transport a reassembled bowl such as the one in 
figure 8, no more than a millimeter thick in places. It has to be taken apart after the 
field record photograph, which is why more care is not taken in the initial assembly. 
Getting everything out whole is certainly something we do not expect and something we have 
certainly not achieved. That is why the first field photographs are of such prime importance. 



184 




Figure 7. Plastic envelopes do not withstand the heat of the area. 



185 




Figure 8. Bowl is assembled 
at the site, photographed 
and then unassembled for 
transporting out of the 
field. 



Surprises for the field archeologist do not always come before those of the conservator. 
For the last five years, we have been finding a great many coins of the Islamic dynasties of 
this region belonging to the tenth to thirteenth century. No gold coins have been found, 
but they do exist. We have many coins with oxidized surfaces we believe to be silver (fig. 
9) because they so closely resemble in appearance known silver coins of the 3rd-century 
Sasanians who also lived in this region. Even larger quantities of copper coins have been 
found, singly or in hoards of up to 406 (fig. 10). We have also found smaller quantities of 
coins we believed to be lead since Sistan did have one of the few known lead coinages (fig. 
11), but this information is quite restricted. Most writers on Sistan coins recognize only 
gold, silver, and copper. In fact, almost all the "silver" and "copper" coins are lead, some 
with a thin copper covering partly intact. All of these coins, kept in standard paper coin 
envelopes and stored in wooden drawers exhibit signs of an alarmingly rapid disintegration 
owing entirely, it seems, to their contact with wood, or paper manufactured from wood pulp 
(fig. 12). Their environment has now been changed, but they can be saved only by treatment 
in the conservation laboratory. It seems to make no difference whether the coin was a 
surface find or recovered from four meters down; the disintegration rate appears to be the 
same. These 700 to 1000-year-old coins appear to have suffered more in the last four years 




Figure 9. Islamic coin be- 
lieved to be silver show- 
ing oxidized surfaces. 



186 




Figure 10. Islamic copper 
coins were found singly 
and in hoards of several 
hundred . 




Figure 11. Islamic coins 
believed to be lead. 



than the preceding hundreds. In their natural environment, if on the surface, they are 
subjected to a daily temperature differential of approximately 20 degress Celsius, with 
summer highs in excess of 55 degrees and winter lows of down to -20 degrees, to intense 
dryness and week-long periods of wet. Those deeply buried, on the other hand, had a constant 
environment. 



187 




I might hazard the guess that no one became a field archeologist through an interest in 
the corrosion of metal, and perhaps in most digs, metal corrosion is not so pressing a 
problem that it cannot wait until objects reach a laboratory. But many objects, and in- 
creasingly so, never become the concern of conservators in museums because they never reach 
museums. They stay in the countries where they have been found, where there are, most 
often, no laboratories, no educated conservators, and often not even an awareness of the 
importance of conservation. This is a problem that is going to require much more attention 
on the part of conservators rather than field archeologists. It is true now, and will be 
ever more so, that the laboratory is going to have to go to the object, not the reverse. I 
cannot think of an archeologist who would not esteem having a conservator as a collaborator 
in the field. Adding an additional person to a dig is not as simple as buying another plane 
ticket and twenty pounds of spaghetti. While that may be the case with the vacation excava- 
tion, it can be more like expanding a laboratory to accommodate more researchers. I know 
that for our project the addition of one person would have required another $300 tent, 
another $8,000 vehicle, more field gear of all sorts and increased volumes of water, food, 
gasoline, and the storage capacity for handling these, all quite apart from that person's 
specific essential work supplies and equipment. To my knowledge, our most enlightened 
granting organizations still frown upon budgetary items for a conservator associated with an 
archeological excavation. They certainly will not countenance sums for restoration of 
cultural properties, which says something about the values of our society today, though this 
attitude is not without its causes. The storerooms of the museums throughout Asia are 
packed with disintegrating objects, some of which may be saved at little cost by a conserva- 
tor who knows what is required. But, the initiative to save these objects is going to have 
to come from those who are capable of saving them, and it is a task for which little money 
is presently available, with a gratification quotient only personal and small, the victory 
still a Pyrrhic one. 



Discussion 

J. Kruger: We have heard several references to the use of vapor phase inhibitors. Has 
anyone ever tried to store lead coins in containers containing these vapor phase inhibitors? 
I do not know whether it would work for lead, but it is very inexpensive and easy to use. 



188 



V. Trousdale: No attempts have been made to save them as it was only within the last few 
weeks that it was discovered that they are disintegrating. First, we have to photograph 
j them and then they will be submitted to the Conservation-Analytical Laboratory for treatment. 
I hope that they will be preserved in such a way that they can still be read. 

W. T. Chase: You remarked that the coins have corroded more in the past six years than 
they had in the previous 600. From your slides of this region, it would appear that 
there is very little wood and other vegetable matter to supply the organic acids to 
attack lead; the more likely source of the present corrosion would be the paper, probably 
kraft paper, of the envelopes and the wood of the storage drawers. 

W. Trousdale: They have been far from wood as there is very little wood in that area. 
] The only kind of wood in that area are alkali bushes. 

j W. T. Chase: Is Mr. Organ here? What about altering the environment to stabilize the 
I lead until treated. Do you have any suggestions along those lines? 

I R. M. Organ: That was an extremely interesting lecture and I appreciate it. The lead 
problem is relatively simple if you have enough space to carry some Perma-life envelopes 
with you. You just pop the lead into those. The Perma-life paper contains a percentage 
of calcium carbonate which will absorb any organic acid on its way to the coin. That is 
the critical moment. Once organic acids get to the lead they stimulate corrosion and 
continue it. Once back in the lab, of course, we can deal with this. We can regenerate 
the material in exactly the position that it is now. The other thing you can do in a 
, hurry is to put shellac varnish on it. That will hold everything in place so that you 
' can still read them and we can still work with them and get them back for you afterwards. 

Af. Goodway: This phenomenon seems to have been first observed at the British Museum or 
at least the publications imply that. How does one get that kind of observation into the 
kind of literature that the archeologist reads rather than just the literature that the 
conservators read? 

' R. M. Organ: That is a very good question, indeed. Of course, it has been observed for 
! a long time. Plenderleith' s^ first book on conservation published in 1936, I believe, 
showed pictures of moldering lead bullae in wooden drawers. They were photographs so one 
could leaf through the pages and see that this was something of importance. The difficulty 
is that within museums, curators and archeologists don't necessarily come into contact 
with the conservators, or haven't done so in the past. At least, they come in contact 
when they have some particular problem: they come and get it solved and then go away 
again. What they really need is some kind of persistent contact so that they know all of 
the ins and outs of the problem. That is one reason why, in the Smithsonian, we have a 
. lot of lectures on video tape of conservation processes. These are supposed to be 
sufficiently entertaining to persuade people to keep on watching them. A number of 
curators have, in fact, watched these things years ago and I hope that they will come 
: back and watch them again. This is, of course, a problem in communication and it is 
I really very serious. A lot of these metals problems we know all about. There are one or 
i two books now for archeologists in the field. There is an early one by Dowman^ which 
L suggests things that you can do. 



^The latest edition of which is: Plenderleith, H. J. and Werner, A. E. A., The Conservation 
of Antiguities and Works of Art, 2nd Edition (London, Oxford University Press, 1971). 

^Dowman, E. A., Conservation in Field Archaeology (London, 1970). 

189 



i 



STRUCTURED QUESTIONS 



Question 1: What is the smooth lustrous black surface on ancient bronze mirrors? 



] W. T. Chase: This will be a minilecture on mirror-black. The person who should be giving 

1 this is Professor Ursula Franklin of the University of Toronto. She has worked on this 

I problem much longer than I, but she is presently attending the archaeometry meeting in 

• Edinburgh. (Comments contributed by Prof. Franklin will be found at the end of this 

I discussion.) This particular black patina occurs on Chinese bronze mirrors and some 

I weapons from the late Chou Dynasty (about 350 B.C.) until perhaps as late as the T'ang 

I Dynasty (ending in A.D. 907). The questions are: what is this patina; and why is it 

1 black? 

I The material is very corrosion-resistant, but, where it does corrode, a typical warty 

j corrosion can be seen. Figure Q-1-1 shows a mirror now, in the study collection of the 

I Freer Gallery of Art, with four warty areas; one at the rim can be seen in side view. For 

I other views of the warts, see the photograph of the spearpoint (fig. Q-1-12) and figures 
Q-3-3 and Q-3-4 (p. 206). 

Those who studied it before World War II include: William F. Collins, (The corrosion 
of early Chinese bronzes. Journal of the Institute of Metals , 45, 23-55 (1931)); Harold J. 
Plenderleith, (Technical notes on Chinese bronzes with special reference to patina and 

j incrustation. Oriental Ceramia Society, Transactions, 1_6, 33-55 (1938-39)); and R. J. 

I Gettens, (Some observations concerning the lustrous surface on ancient Eastern bronze 
mirrors, Technical Studies in the Field of the Fine Arts, 3_, 29-37 (1934)). For more 
recent studies one should consult the following: Tsurumatsuru Dono, {The Chemical investi- 
gation of the Ancient Metallic Culture, Tokyo, Asakura, 1967); A. G. Bulling and I. Drew, 
(The dating of Chinese bronze mirrors. Archives of Asian Art, 25, 36-57 (1971-72)); I. 
Drew, (Dating and Authentication: Chinese bronze mirrors, MASCA Newsletter, ]_, 1, 2-3 
(1971)); Ursula Franklin, (Chinese black patinas, paper presented at 182nd Annual Meeting 
of the American Oriental Society, Chapel Hill, NC, April 20, 1972). 

Some typical examples of this black patina include the mirrors from the tombs of Liu 
Sheng and Tou Wan, discovered in Lingshan Mountain in the western suburb of Man-ch'eng, 
Hopei Province, China, in 1968. For information of the excavation see Man-ch'eng Han mu 
fa chueh chi yao (A report on the excavations of the Han tombs at Man-ch'eng), Kaogu 
(Archaeology 1 , 1972, 1 , 8-18; a rubbing of one of the mirrors is shown in figure 14 on 
page 17. See also Ku Yen-wen, Han Tombs at Mancheng, l^ew Archaeological Finds in China, 
Foreign Languages Press, Peking, 1973; Hsiao Yun, Man-ch'eng Ran mu chu tu ti tso ching 
ying niao ah'ung shu t'ung hu (Two bronze vases with gold inlaid bird-tracks inscriptions 
found in Man-ch'eng), Kaogu, 1 972 , 5, 49-52; Man-ch'eng Han mu ' ching -lou-yu-i ' ti chin li 
han fu yuan (The cleaning and restoration of jade suits found in Man-ch'eng), Kaogu, 1972 , 
2, 39-47; Jan Fontein and Tung Wu, Unearthing China's Past, Boston, 1973, pp. 100-102; 

j Historical Relics Unearthed in lHew China, Foreign Languages Press, Peking, 1972, p. 94 

1 ff . ; Wen-hua-ta k'e-min ch'i chin, Ch'u-tu wen-wu. Wen Wu Press, Peking, 1972, p. 25; 
The Genius of China, an exhibition of archaeological finds of the People's Republic of 
China held at the Royal Academy, London London, 1973; p. 99 ff . ; The Chinese Exhibition 

and The Exhibition of Archaeologiaal Finds of The People's Republic of China, both Washington, 
D.C., 1974, pp. 65-71 and pp. 31-33, respectively. Figure Q-l-lA shows three of the mirrors 
from these tombs on exhibition in Peking during 1973. The mirrors are large; the middle 
one in our plate is about 21 cm in diameter. Two of the three have a fine black patina. 
It appears to have been fairly dry in this tomb. Objects with other patinas appear from 
these tombs as well (see Question 6). Some of the later Han mirrors have a much more 

I silvery look to them; they may not be the same material or patina. This is another question 

I for study; is there more than one type of mirror black patina? 



191 




Figure Q-1-1. A Chinese bronze mirror dating from 
about 200 B.C. Ex-collection James M. Plumer 
(JCP 2), now Freer Gallery Study Collection SC 551 
This mirror is 8.9 cm in diameter (3 1/2 inches), 
and is covered with a deep-green to black patina 
with four bright-green warty spots. The front 
surface is shiny black with numerous warty areas. 




Figure Q-l-lA. Three mirrors from the Man-ch'eng tombs (see text for references). The 
large mirror on the right has a particularly fine black patina. See color plate o. 



This patina also occurs on weapons, often on weapons with pattern decoration from the 
late Chou and Han Dynasties. The pattern may be black or part of the pattern may be black 
and part may be green. This sword (fig. Q-1-2) in the collection of the Hong Kong City 
Museum and Art Gallery has a shiny black blade and a matte green haft. No joint can be 
discerned between the blade and haft. The weapon, from the tomb of Liu Sheng (fig. Q-1-3) 
has black spots on a bronze ground. A bimetallic sword in the Shanghai Museum (fig. Q-1- 
4) is very similar to the one shown by Cyril Smith. The analyses listed in the figure 
caption show that the outside has a higher tin content. The tin content varies from the 
outside, blacker area to the inside, greener area. One can form an idea of the hardness 



192 



Figure Q-1-3. A bronze dagger-axe {ko) from the Man-ch'eng tomb 
of Liu Sheng, aa. 20 cm in length. The object is bronze-colored 
with black spots; the mount for the shaft is gilded. 




7g48;^ I9.8B/ m 0.25/ 



Figure Q-1-2. A bronze sword from 
the late Chou period (or Warring 
States period, 4th to 3rd century 
B.C.). This sword has a shiny 
black blade and a matte green guard 
and handle. No joint could be seen 
between the two, (From the collec- 
tion of Hong Kong Museum of Art, 
Urban Council, Hong Kong.) 



Figure Q-1-4. A fragment of a late Chou sword with 
analysis of the two metals from which it is made, 
as displayed in the Shanghai museum, People's 
Republic of China. The outer portions have the 
composition 78.48 percent Cu, 19.88 percent Sn, 
and 0.25 percent Pb. The inner or central portion, 
which is green in color, contains 79.70 percent Cu, 
8.44 percent Sn, and 10.15 percent Pb. The 
analyses were made by the Shanghai Metallurgical 
Research Institute. 



193 



and brittleness of the metal by the fracture along the edges. Further examples of these 
various types can be seen in the Brundage Collection, the Winthrop Collection of the Fogg 
Museum, the Singer Collection, and many others. 

Since this sort of patina almost invariably occurs on high-tin bronzes, it is worth 
looking at the occurrence of high-tin bronzes in China. Figure Q-1-5 shows a ternary plot 
of some of our analyses of Chinese bronzes. As you can see, the mirrors form a distinct, 
high-tin group. We have not yet included weapons in this diagram. 



cu 

100 % 





PB " > SN 

"50% 50 ^ 

Figure Q-1-5. A ternary plot of Chinese bronzes analyzed at the Freer Gallery of Art. The 

mirrors (represented by $ here) form a distinct, high tin group. 



Now to return to the black patina; the analytical work done by Ursula Franklin at the 
University of Toronto and by myself at the Freer, has failed to get any x-ray diffraction 
pattern for this black material at all. I have put a piece of mirror in the x-ray diffrac- 
tometer and got only a pattern of distorted alpha-bronze. The black areas seem to give no 
x-ray diffraction pattern at all. In x-ray fluorescence analyses done by Maurice Salmon in 
the Conservation-Analytical Laboratory of the U.S. National Museum, we found increased 
concentration of tin, lead, and iron at the surface. 

When microprobe analysis was attempted (with the assistance of Charlie Obermeyer, 
Mineral Sciences, U.S.N.M.), the tin trace (fig. Q-1-6 and Q-1-7) showed an extra concentra- 
tion of tin at the surface. Iron and silicon also concentrate at the surface (figs. Q-1-8 
and Q-1-9). It appears to be an iron-tin-silicon diffusion into the surface. 

We often see in cross-sections that the outer layer is quite transparent, green or 
greenish-black, and glassy in appearance. Under the transparent, colored layer lies the 
uncorroded delta phase of the metal, which can usually be seen partly down through the 
transparent material. The transparent material also shows a conchoidal fracture and looks 
very smooth under the scanning electron microscope (fig. Q-1-10). The warts, shown in 
figure Q-1-1 above, often lift this transparent, colored layer up; it can be seen in flakes 
in the warts. 



194 




(c) 

Figure Q-1-6. Three photomicrographs of the surface layers of a Chinese bronze mirror dating 
from the late Chou period. (JCP 11; FGA SC555). The remarkable polished thin section 
across the thickness of the mirror from which these photographs were taken was made by 
Grover Moreland of the Department of Mineral Sciences, U.S.N.M. The photographs match the 
the electron microbeam probe photographs below in magnification (figs. Q-1-7, 8, and 9). 
(a) shows, in bright field reflected light, the dark grey layer of the glassy black patina 
on the outer (upper) surface. Under this is a lighter grey, partly mineralized layer, and 
then the bright, uncorroded metal, (b) shows the same area in a combination of bright field 
reflected light and transmitted light illumination. The dark rectangle at the top is the 
remains of damage caused by the electron beam in the probe, left here for location. The 
sample was repolished after the probe work was done. The upper, glassy layer of the patina 
can be seen to be quite transparent, and not completely continuous, (c) shows the same area 
in dark field reflected light. The upper surface of the patina can be seen to be quite smooth. 



The spear point shown in figures Q-1-11 and Q-1-12 was given to the Freer Gallery by 
I Sir Harry Garner for study purposes. It is shiny green in tone, with a diaper-pattern 
I decoration in brown. The warty corrosion concentrates along the mould joint line at the 
! central rib, around the tip, and along the edge of the blade. These are the areas in which 
there is the greatest chance of mechanical damage to the protective patina during use. 



195 





Figure Q-1-7. Tin concentration in the area 
of figure Q-1-6 (a,b,c) as revealed by the 
electron microbeam probe. The cracks in 
the specimen show vaguely. The outer sur- 
face and the corrosion penetration zone 
are enriched in tin. 




Figure Q-1-8. Iron concentration in the 
same area as Q-1-7 revealed by the electron 
microbeam probe. The iron concentrates in 
the glassy surface layer. 






Figure Q-1-9. Silicon concentration in the 
same area as Q-1-7, by electron probe. 
Silicon concentrates in the outer surface, 
but also appears to be diffusing inwards 
into the mineralized zone. The lower edge 
of the mineralized zone (as seen in Q-1-6 
(a)) can be seen to be the lower boundary 
of the silicon diffusion. 



Figure Q-1-10. A photograph of the surface 
and fractured edge of mirror JCPll, taken 
with the scanning electron microscope 
(Walter R. Brown, National Museum of Natural 
History). The outer surface can be seen, in 
the left portion of the photograph, to be 
smooth and somewhat cracked. At the center 
of the photograph the fracture between the 
outer surface and the mineralized zone under- 
neath it can be seen (arrow). The mineralized 
zone has fractured roughly. On the right can 
be seen the sawcut where the sample used for 
the polished thin section (above) was taken. 
Magnification 2/3 that of figure Q-1-6 above. 



196 



Figure Q-l-ll. A Late Chou Dynasty spearpoint, given 
to the Study Collection of the Freer Gallery or Art, 
by Sir Harry Garner (SC-B-88). Length = 24.4 cm. 
Much warty corrosion can be seen on the piece, con- 
centrating along the sharpened edges of the blade 
and at sharp corners on the median rib and shaft. 
The sides of the blade are decorated with a diaper 
pattern in a brown color. 




Figure Q-1-12. A detail of figure Q-l-ll, 
showing more clearly the warts and the 
decoration. 



197 



The brown-and green decoration on the surface may well be etched. The green may be 
the same sort of material as the black mirror patina. It has a glassy appearance. This 
decoration is probably an example of etching for decorative purposes such as Cyril Smith 
has mentioned. The cutting edge of the blade has corroded so that the center part of the 
dendrites has remained, but the alpha-delta eutectoid has been removed. Redeposited 
copper and a thick corrosion crust can also be seen (fig. Q-1-13). 

Seen in more detail (fig. Q-1-14) there are two types of corrosion; in one the delta 
phase is removed, and in the other the alpha is removed. Further from the blade edge you 
can see this effect more clearly (fig. Q-1-15 and Q-1-16). The outer surface is at the 
top of the figures. The outer surface of the patina (which I believe in this case to be 
really an artificial patina) is very smooth. Beneath this can be seen the uncorroded 




Figure Q-1-13. A bright field metallograph of the blade edge 
of the spearpoint, showing corrosion with removal of the 
alpha-delta eutechtoid in the triangular area of the cutting 
edge. Magnification ca. XI 6. 




Figure Q-1-14. The same area as Q-1-13, detail in the center 
of the blade edge; magnification ca. X48. 



198 





Figure Q-1-15. Photomicrograph of a section taken of the 
spearpoint in figure Q-1-11 shows the area in bright field 
illumination. The sharp line of the shiny patina on the 
outer surface shows at the top; below this lies an area 
which has been mineralized by the removal of the alpha- 
phase, leaving shiny patches of delta phase. At the center 
and lower right, an area can be seen which has corroded 
inwards from a crack at the right. This area has corroded 
with removal of the delta phase, leaving the alpha. Voids 
and patches of cuprite can also be seen. Magnification 
ca. X48. 




delta phase. The sides of the sample were formed by cracking in the spear point. Corro- 
sion has worked in from the sides; this corrosion has removed the delta phase first. The 
common type of corrosion that we see in ancient Chinese bronzes usually proceeds by 
removal of the delta phase first. We can see both the usual type and the inverse type on 
this one sample. My contention is that the corrosion on the outer surface was synthetically 
produced, and where it has chipped or cracked, normal corrosion has proceeded. 



199 



The outer surface at higher magnification shows that, in the alpha-delta eutectoid 
under the outer surface, the delta phase remains, but the alpha phase has corroded away 
(fig. Q-1-17). One can see a very similar structure on the mirrors, except that it is 
black instead of green (although sometimes it may be green as well). Leaving aside the 
question of how the pattern-etching or mineralization was done, which we would also like 
to know, how was that shiny black or green patina produced? 



The process has to remove the alpha phase of the bronze, possibly precipitating tin 
oxide (which I think may be at least one component of the outer layer); it must deposit 
pseudomorphical ly with the dendrites that were there; and the iron and silicon diffusion 
into the surface have to be explained. Ursula has said that the temperature of the process 
cannot be much above 300 °C or one would see changes in the metallography of the objects, 
which she has not observed. What is the process, and how can we duplicate it in the 
1 aboratory? 

One feature that may be of interest to the industrial people here, and might be 
commercially applicable, is that this patina has done an awfully good job of stopping 
corrosion on the mirrors for 2000 years. 

T. D. Weisser: I do not know what the black substance is. In doing my experiments on the 
effects of 5 percent sodium sesquicarbonate at room temperature on various compositions of 
bronzes, I noticed a similar surface effect of a black, smooth, lustrous material on a 
modern, 80 percent copper-20 percent tin polished bronze sample after it has been in the 
solution for about 24 hours. The microstructure of my sample is similar to your sample, 
and the alpha phase is preferentially removed in the sodium sesquicarbonate, while the 
delta phase eutectoid is left behind. The composition of my sample is almost identical to 
the composition of the outer layer of your Chinese bronze example. Perhaps the preferential 
attack of the alpha phase is due to artificial patination with something such as cyanide 
or ammonia or a double carbonate such as sodium sesquicarbonate. Or perhaps it is due to 
a burial situation setting up similar conditions. 

w. T. Chase: I am bothered because I am not so sure it is artificial; were the Chinese so 
sophisticated in 200 B.C.? So we may be seeing a combination of natural and artificial 
processes, and will have a hard time sorting out which is which. 




Figure Q-1-17. A photomicrograph of the spearpoint 
in figure Q-1-11 showing the outer surface and 
mineralized zone at higher magnification [oa. X300) 
and bright field. The remnant delta, quite un- 
corroded, can be seen in the central portion of the 
photograph. 



200 



M. Pourbaix: Terry has just said that something very similar to what I have in mind. 
Assuming that the patina existing on these mirrors is mainly amorphous hydrated tin oxide, 
it might be possible to produce it artificially by dealloying in some chloride-free running 
solutions, without stagnation preferably. Such thin films might be studied with Auger 
spectroscopy. Has this been done? 

W. T. Chase: No. 

C. S. Smith: The black finish on Chinese bronzes was probably intentional, for, as on the 
Brundage short sword mentioned by Mr. Chase, it is terminated at a specific point in the 
design, not irregularly as it would with natural corrosion. The reaction that produced it 
was an unusual one, for the alpha phase was completely replaced with an exact pseudomorph 
in reaction product, while the delta phase remains completely untouched. Even the fine 
alpha lamellae within the eutectoid are changed within a matrix of uncorroded delta. The 
surfaces of the delta particles (visible under the microscope through the somewhat transparent 
structureless corrosion product) are shiny bright and retain all the detailed surface 
contours left by their formation. There are no visible channels for corrosion around it 
at all. 

The contemporary swords with geometric patterns resulted from a similar but probably 
not identical chemical treatment. On a sample in the Fogg Museum dendrites, or rather 
dendritic pseudomorphs after alpha, can be seen extending uninterruptedly between the two 
regions, now appearing as two different kinds of corrosion product. 

C. E. Birahenall: Silica can provide the basis for a glassy layer, but to be low melting 
borax and phosphate additions might contribute. Have boron and phophorus been looked for 
in the analyses? 

w. T. Chase: I think phosphorus has, but not boron. I hope to do a materials balance 
analysis to see if we have most of what is in there. It is hard to get enough material to 
do wet chemistry because the layer is always so thin. 

C. s. Smith: It is very thick as patinas go. 

J. Kruger: Why not use electron diffraction to identify the black material? Even though 
it may be amorphous to x-ray diffraction one can sometimes see small crystallites with 
electron diffraction. 

W. T. Chase: I might add that during electron microprobe analysis we observed light emission 
as the beam traversed the black area, a definite blue glow. It is very non-conducting. 

J. N. Andre: The absence of x-ray diffraction patterns using the mirror as sample may be 
related to an amorphous compound, but a preferential orientation of the film can also 
perturbate the result considerably and cause diffraction peaks to be weak or missing. It 
can happen when there is a preferential growth direction of the product film or a kind of 
epitaxy with a substrate having a fiber structure; diffraction on a powder sample removed 
from the surface would probably be free of this problem. 

L. van Zelst: A black patina like the one discussed has been observed in Islamic high tin 
bronzes from the Near East. In general, the problem has always been assumed to be related 
to high tin bronzes, with perhaps amorphous hydrated tin oxide in the layer. 

Recently I found it on two "Luristan" bronzes, one with 18.7 percent tin but the 
other with much lower tin content (approximately 9.3 percent). The metallurgical examination 
revealed that the mechanism of the corrosion was the preferential attack of the alpha 
phase, as it is for mirror black. It has in general been the opinion that this was an 
artificial patina. In the case of the "Luristan" bronzes, however, the possibility of 
natural patination must be considered. An outline for a possible mechanism was suggested 
by Richard Stone. It involves changes in pH in the burial conditions, starting out with 
slightly acidic soil, preferably high in ferric ions (remembering that there is an enriched 
iron content in the top layer) which might preferentially attack the alpha phase, and then 
j later the pH changes and becomes basic and freezes the whole system and even will help 
\ precipitate silica, which is pretty mobile during most neutral conditions. 



201 



w. T. Chase: We hope to get this material into print so that people can question and 
discuss it. More input of ideas as we have had at this conference will be very valuable. 
It may be that someone has had experience in producing corrosion films like these on high- 
tin bronzes; we would like to hear of their experiences. 

C. S. Smith: I have two more points to add. It seems that the fine black only occurs on 
high tin bronzes, yet it is the alpha phase that has been corroded, and the alpha phase 
has the same composition regardless of how much tin there is above, about 10 percent. 

In both the Brundage sword and the similar one in the Fogg Museum, the alpha dendrites 
are continuous from the black areas to the green areas, and you can see the shape of the 
dendrite in both the dark and the light green patina. 

I find it impossible to believe that these are accidental patinas; their outlines have 
all the earmarks of something done intentionally. 

w. T. Chase: I have a thesis, with which Ursula Franklin does not agree, that these 
surfaces may have been initially shiny metal color with some very light surface treatment 
which is then partly corroded and partly stained to black or deep green. 

There is a sword, not yet examined that I know of, from the Man-ch'eng tombs, which 
was in the Chinese archaeological exhibition [The Genius of China, p. 160, no. 160; The 
Chinese Exhibition, p. 70, no. 160), which has a glassy, bronze-col ored surface. In the 
case in the exhibition, this surface had a very strange and glassy appearance; I've never 
seen another one like it. Perhaps these things had colors like this sword or glassy- 
silvery surfaces; due to staining or other things, over the course of time the surfaces 
may have turned to the black and deep greens. As I said before, the effect may be a 
combination of natural and synthetic patination. 

J. s. olin: For a non-crystalline material the use of infrared spectrophotometry is 
often helpful. If the black surface referred to does not give an x-ray diffraction pattern, 
it is possible that the anion might be identified using infrared spectrophotometry. 
There are characteristic peaks for the silicates and phosphates which have been suggested 
as possible compounds. 

C. s. Smith: The uniform appearance of the black patina on the forged high-tin Iranian 
bronzes when seen in section under the microscope is much more like that of a high-tempera- 
ture oxidation layer than one produced by electrolytic corrosion. There is no deep selective 
penetration into the alpha phase or along grain boundaries such as occurs in the cast 
Chinese bronzes. 

R. M. Organ: Does it have to do with tin sweat? 

W. T. Chase: The difficulty is the iron and silicon content, not the tin content. Most 

of these bronzes, especially the mirrors, have a very high polish, some a very high selective 

polish. Such abrasive polish (not a burnish) would have removed surface tin. 

B. F. Brown: Has anyone determined the refractive index of the black layer? 

(No response) 

Ursula Franklin: In the following paragraphs you will find a few additional comments that 
I would have made, had I been present; all of my remarks are based on experimental work 
with black mirrors, since I never have had a chance to work with a "pattern-etched" sword. 

(1) The fact that most mirrors are high-Sn bronzes (chosen because of colour, hardness 
and ease of polish) and that some of these mirrors have a black surface finish (again in 
my opinion for functional or decorative reasons) should not drive us to the conclusion 

that the black finish depends chemically on a high Sn content. After all, it is the a-phase 
that is altered. 

(2) All I have seen has convinced me that it is a purposefully executed surface 

finish. Look at three areas of evidence: (1) its chemical inertness, (2) its microstructure, 
(3) its external application on the mirrors. 



202 



With regard to point (1) the surface layer is an amorphous non-conducting oxide or 
silicate layer, amorphous even to electron diffraction as far as we can see; it is not an 
isomorphous film of preferred orientation. If one were to assume the formation of a 
protective film formed by some type of preferential corrosion (as in the case of stainless 
steel) one would expect to find the film enridhed in certain constituents, that have 
diffused preferentially to the surface to form the *protecti ve film (such as Cr in stainless). 
Below the surface of the film one might find a depleted layer until one reaches the 
equilibrium bulk concentration. In the black surface finish, however, the elements present 
are not part of the alloy. Si and Fe is present in the surface but not in the bulk, Sn is 
enriched in the surface but not noteably depleted below. We have lots of microprobe 
traces, taken on cross-sections, so that the distribution of Cu, Sn, Fe, Si, S, 0, etc., 
can be documented quite consistently. If the black surface of the mirrors had been produced 
by an accidental reaction with a specific environment, as Collins thought, it is difficult 
to see why this reaction would stop at a certain point, leaving a chemically inert, tightly 
adhering and insulating surface on the object. Furthermore, when interaction with the 
environment occurs, resulting in the "warty" corrosion, it is the bronze below the surface 
layer that reacts, not the black surface itself. 

Point (2). Looking at cross-sections, one has two overriding impressions. One is 
the very gradual transition between the altered surface layer and the bronze proper. 
There is no abrupt change in the micro constituents, in their orientation or their size, 
no discontinuity of any sort. One sees occasionally cracks with altered inner surfaces, 
but the general impression is one of a partially etched sample. Secondly, it becomes 
clear that the non-metallic translucent layer is on the very outer edge; it is quite thin 
and never penetrates very deeply. When this layer is broken, corrosion products such as 
malachite can be seen beneath the surface. 

I have not seen any electrochemical interaction between the bulk alloy and the altered 
layer, for instance a reaction of the type that one observes at the contact of dissimilar 
metals. 

Point (3). Looking at black mirrors one can often see polishing marks and scratches on 
the non-ornate side helow the finish. The loop on the decorated side shows black surfaces 
on the inner and on the outer surface as well as on the mirror under the loop (see sketch in 
fig. Q-1-18). It would be difficult to imagine that this would happen randomly. In cross- 
section, one can see that the altered layer follows the topography of the ornamentation. 



Figure Q-1-18. Idealized section through 
loop of mirror--black surface present on 
top and bottom and around loop. Black 
surface shown here by dots. 




We have checked carefully for any influence of Hg or S. While some mirrors contain 
small amounts of S, it is in the form of sulfide inclusions, more or less evenly distributed 
through the casting. We found no Hg. 

Until I know more, I think that we have to envisage a two-step process. First, some 
type of sedimentation or etching, that attacks the a-phase, basically replacing Cu by Fe 
(or its compounds). This might be followed by a process that involves temperature and a 
glazing or enameling reaction, which produces the stable outside surface and the gradual, 
fairly deep transition zone which I have called "the altered layer." (It may be that the 
thickness of this layer has increased with time.) 

During this phase the Sn might have migrated to the surface, producing a low melting, 
mixed oxide as the coating and the Sn enrichment in the altered layer. Of course, the two 
steps could be part of one process, akin to Wulff's explanation of the Persian bead making. 
But whatever it was, I am convinced that it is an intentional process applied to the fully 
finished and polished object. 



203 



Question 2: What was the patina on outdoor bronze statuary during Roman times? 



J. Kruger: Brochantite. 

B. F. Brown: Vesuvius would have been a copious source of sulfur (as might also open sewers). 

p. Weil: It appears that prevailing practice during Roman times was to gild major outdoor 
bronzes. Recalling Pliny's remarks that the ancients {i.e., the ancient Greeks) used to 
coat their statues with bitumen to protect their luster, it was all the more surprising 
that the Romans came to gild them to achieve the same effect. The reason for gilding 
appears to be that with gilding no maintenance would be required. 

R. M. Organ: The polished copper might have become tarnished from hydrogen sulfide. 

After all, there were people there. Then the copper sulfide could oxidize up to brochantite; 

perhaps they were green if neglected. 

w. D. Riahey: The question of sulfur balance in the air is complex. There are natural 
and everpresent sulfur sources. Kellogg, et. at. (W. W. Kellogg, R. D. Cadle, E. R. 
Selen, A. L. Lazrus, and E. A. Martell, The Sulfur Cycle, Science, 175 587-596 (1972)) has 
attempted to provide a sulfur balance. Man provides much less than half the sulfur burden 
from fossil fuels. Some sulfate is postulated to be aerosoled from seas and bodies of 
water and so would have been present in antiquity. 

R. M. Organ: There were sulfur springs... 

C. S. Smith: ...and sulfur from copper smelting. 



Question 3: What is the black corrosion product associated with the initial intergranular 
attack of bronzes? 



w. T. Chase: The question was asked by John Gettens in his article, "The Corrosion products 
of an ancient Chinese bronze" {Journal of Chemical Eduction, 28, 68-71 (1951)). His 
statement of the problem is worth quoting. He is describing the corrosion layers on the 
"Kelley Bronze", a broken and much repaired Chou dynasty bronze vessel of the type hL[, 
presented to Gettens by Charles Fabens Kelley, then Curator of Oriental Art at the Art 
Institute of Chicago. Dr. Kelley gave Gettens permission to cut sections and use the 
bronze for technical study with no restrictions. The fragments of the bronze now reside 
in the Study Collection of the Freer Gallery of Art, with the number SC548 (see also, R. 
J. Gettens, The Freer Chinese Bronzes, Volume II, Technical Studies; Smithsonian Institution, 
Freer Gallery of Art, Oriental Studies Series, No. 7; Washington, 1969, p. 127 and p. 183). 

The corrosion zones and the several mineral layers were of chief interest. 
Most striking is the intermediate penetration zone where the eutectic (tin-rich 
phase) has been converted to a black product leaving dendrites of alpha bronze 
(copper-rich phase) unattacked. It is a natural etch. In the casting this 
bronze has apparently cooled slowly, allowing alpha dendrites to form freely and 
nearly perfectly (fig. 3-2). This zone of interphase penetration is generally 
1 to 3 mm thick, but in places it goes entirely through the metal. Because it 
is so intimately mixed with uncorroded alpha it was quite impossible to isolate 
the black material for microchemical tests. It was possible, however, from a 
polished section, to isolate chips free from outer corrosion product and to 
subject them to x-ray diffraction analysis. The powder pattern showed mainly 
an expanded copper lattice, but in addition it showed faint lines characteristic 
of cassiterite and also one unidentified line. No lines of the copper chlorides 
were observed. Tin oxide could be expected, and indeed, white tin oxide 
is plentifully seen in the outer corrosion layers. The faintness of the 
cassiterite lines indicate that the tin oxide is in a cryptocrystall ine state. 
Why does it seem to be black here? It is possible that it is stained black with 
cupric oxide, and it is well known that some forms of cupric oxide (like 



204 



melachonite) are so nearly amorphous that they give undistinct diffraction 
patterns or none at all. If the black is cupric oxide it is doubtful if it 
is formed by direct oxidation of copper especially under the reducing conditions 
that seem to prevail in the interior after corrosion attack is well along. 
There is a remote possibility that the black is formed from cupric hydroxide 
precipitated at cathodic areas of local cells set up by electrolytic action 
in the salt environment. J. W. Mellor (^4 Comprehensive Treatise of Inorganic 
and Theoretical Chemistry, Volume III, p. 144, London, 1923), cites evidence 
that cupric hydroxide hydrogel in contact with water or salt solution can 
eventually dehydrate and turn black even below room temperatures, although 
several months may be required to bring about the change. Bengough and May 
(Seventh report to the corrosion research committee, J. Inst. Metals, 32^, 
103, (1934)) state that in copper corrosion dark brown or black hydrated 
cupric oxides are important constituents of scales that form on the copper 
surface. This explanation of the initial black corrosion product is, however, 
highly conjectural since we have so little direct analytical information 
bearing on it. 

The Kelley bronze has the general corrosion structure shown in figure Q-3-1 . A more 
detailed view, figure Q-3-2, shows the intermediate penetration zone that we are interested 
in, along with cuprous chloride and cuprite. What is it? 




Figure Q-3-1. Cross-section of the wall 
of the Kelley Bronze (FGA SC-B-18), 
magnification aa. XIO.. From the top 
can be seen; a thick corrosion crust 
(dark); a layer or redeposited copper 
(broad, bright line); a thin line or 
corrosion product, possible cuprous 
chloride (dark); the original metal, 
now corroded with the al pha-dendrites 
remaining, and surrounded by the black 
material in question; and more layers 
of redeposited copper and corrosion 
products. 




Figure Q-3-2. Another metallograph 
of the Kelley Bronze, magnification 
ca. X65, bright field, showing the 
remnants of the alpha-dentrites 
surrounded by the black material. 



A similar sample comes from this later Chou {ca. 400 B.C.) tube (fig. Q-3-3). A 
cross-section of the whole tube, near the break (fig. Q-3-4), shows one of the warts of 
corrosion often seen on these bronzes. You can see some of the black product on the 
inside of the ring of the tube. On the outside is some of the greenish, glassy material 
that may be an artificial corrosion product (see Question 1). In bright field, the cross- 
section through the tube wall shows the outer layer at the left. You can see the uncorroded 



205 



Figure Q-3-3. Bronze tube, dating 
from the late Chou period (FGA X71), 
aa. 10 inches long overall. 




Figure Q-3-4. Cross-section of a wart 
in the sidewall of tube (fig. Q-3-3); 
the section also shows the entire 
circumference of the tube. 




Figure Q-3-5. Cross-section through 
tube (fig. Q-3-3), bright field 
illumination, unetched, magnifica- 
tion ca. X30. 



delta phase (fig. Q-3-5). It is interesting that the delta phase is the only uncorroded 
metal here. The sample is unetched; the only etching seen on the cross-section comes from 
the corrosion which the sample has undergone. 



206 




Figure Q-3-6. Cross-section through tube Figure Q-3-6A. A detail of the patina on 

(fig. Q-3-3), dark field, magnification the inner surface of the tube, dark 

oa. X30. The sampling hole in the field illumination, ca. X60. 

cuprite layer near the upper left corner Compare with Q-3-6. See color plate j. 
of the picture is 130 ym long and 50 ym 
in width. 



The corrosion has worked through from the inside of the tubes in a more normal fashion. 
Let us examine the inside in some detail. Figure Q-3-6 and figure Q-3-6A show some lovely 
azurite, cuprite, and then this black stuff. 

Using a technique developed by Dr. Walter McCrone of the McCrone Research Institute 
in Chicago {Techniques, Instnments and Accessories for Microanalysts, A User's Manual, 
eds., Walter C. McCrone and R. I. Johnson, McCrone Associates, Chicago, 1974, p. 61; The 
Particle Atlas Edition Two, eds., Walter C. McCrone and John Gustav Delly, Ann Arbor, 
Michigan, 1973, Vol. I, pp. 225-227), we have made tungsten needles with a diameter at the 
point of a few micrometers. Five samples were removed from the marked areas on figure Q-3-6. 
The layer of interest was powdered with the needle, after coating the sample with a thin 
layer of collodion. The needle was stuck through the collodion to dig into the corrosion 
layer. The collodion was again dampened with acetone and allowed to dry. This incorporates 
the particles into the collodion film. The film was then cut with a clean needle, and 
picked up on a glass fiber for x-ray diffraction. By applying a small drop of acetone 
to the collodion plus sample on the fiber, the collodion will ball up at the end of the 
fiber and hold the sample there for diffraction. This way you can sample the individual 
corrosion layers quite precisely. Camera exposures are of the order of 20 hours. 

Diffraction has been done on samples from sites 1 and 2 at the top of figure Q-3-6. In 
the sample from site 1, I penetrated a little into the cuprite layer and all I got was a 
weak but classical cuprite pattern. Site 2, the red layer in the color plate, gave us a 
stronger cuprite pattern. If there is anything else in the sample from Site 1 other than 
cuprite, it is amorphous; at least it did not give us a pattern. 

J. Kruger: Your discovery of weak cuprite lines suggests comparison with the stress 
corrosion of brasses which in ammonia solutions yield black corrosion products that do not 
look at all like cuprite but yield CU2O x-ray patterns. Those have zinc in them; this may 
be cuprite with tin in it. Perhaps your weak cuprite lines are not accidental. 



207 



W. T. Chase: Can this black product of stress corrosion be turned red? 

J. Kruger: No one has done it. (Perhaps it requires a bronze treatment.) 

R. M. Organ: Very fine comminution may make it black--a very fine particulate? 

C. S. Smith: Only if it is an electrical conductor. 

H. M. Organ: Well, cuprite is a conductor. 

C. E. Birahenall: Something ought to be said about the copper sulfides. At low temperatures 
chalcocite, Cui.gg-z has a slightly variable composition, and there are two other phases 
Cu^i geS, djurleite; "and digenite. At high temperatures only a single broad phase exists, 
whTch spreads from about Cu^i.eS to CU2S at high temperatures. Because of the variable 
compositions sharp x-ray diffraction lines are not always obtained. Furthermore, the copper 
diffusivities are high, or they are in Ag2S, so the rate of sulfide growth can be fairly 
high on copper. The cuprous sulfides are predominately black in color. 

P. Caspar: The deeply colored amorphous substance might possibly reflect the intimate 
association of several valence states of copper--either Cu^^ and Cu^ or possibly Cu^ and 
Cu°. It is well known that association of two valence states of the same element leads to 
deeper colors than either valence state alone. 

R. M. Organ: All the other minerals in that area are in the lower valence state.. 



Question 4: What is the composition of the thin black layer between the metal and cuprous 
chloride in many ancient bronzes? 



R. M. Organ: This question has already been spoken to above. However, I would wish to 
remind conservators that regardless of the identity of this thin black layer the source of 
"bronze disease" is the cuprous chloride below the crust of cuprite and above this black 
layer on the metal--all in an area of lower valency. I have always thought of the black 
as resulting from the fine state of subdivision. 



Question 5: How do artificially produced corrosion products differ from naturally formed 
corrosion products, if at all? 



L. Fitzgerald: Artificial and natural patinas on copper cover a range of colors through 
brown and black to a green weathered appearance. Sulfides and oxides are typical com- 
positions of natural patinas that are brown or black. The green natural patinas have 
already been discussed. 

Artificial patinas are not limited to sulfides and oxides; a much broader range of 
compositions is possible. Today even selenides are used. Most of the chemical techniques 
for putting down a patina do not necessarily relate to naturally formed corrosion products. 
However, in all cases we are looking at de-zincification or some other enhancement of the 
copper layer as a first step in getting a good black or dark patina, whether done naturally 
or artificially. 

Secondly, the normal chemical reaction is similar to that used to form INCRApatine 
II, mentioned before, where an oxidation was used to go through the cuprous-cupric states, 
and then a copper salt introduced to give a brochantite. If we are going strictly to 
black or dark layers, today this is done principally by selenides. For a fast black the 
common technique is to use a selenide at a low pH of 3 to 4, giving a very effective 
surface on the bronze. This of course, does not relate in any way to a naturally formed 
corrosion product. 



208 



You can get any color you want from copper metal. Experiments have shown that a full 
I range of colors are obtainable through the use of selenide surface treatment. These colors 
|j range from yellows and reds through blues to dark purples and black. 

J. Kruger: By making statistical studies of many patinas one could perhaps separate the 
j different groups using many different techniques of analysis or different techniques of 
' patination. For example, consider the following: (1) ion probe spectroscopy to get 
concentration profiles; (2) determine electrical properties, p or n character of semi- 
conductive patinas; (3) optical constants by ell ipsometry; and (4) mechanical properties, 
such as hardness. 

I R. M. Organ: To follow up these suggestions, refer to the three reports of Mrs. Kalish, 
Moscow: Examination of the Protective Properties of the Natural Atmospheric Patina on 

I Bronze Monuments, Madrid Conference, 1972; Investigation on Protective Properties of 
Artificial Patina on Bronze Artifacts, Venice Conference, 1975, made to the Conservation 
Committee of ICOM, comparing the mechanical properties, i.e., the ability of the patina to 
be bent around a radius, wear of the surface, and the resistance of the surface to pollutants 
(ammonia, acetic acid, sodium chloride). These reports can be obtained from the International 

I Centre for Conservation, 13 Via di San Michele, Rome 00153, Italy, for ten cents per page. 

P. Weil: Attention is called to the bibliography on The Composition and Structure of 
Natural Patinas by S. Lewin and S. Alexander, published in Art and Archaeology Technical 
Abstracts, 6, (4), December 1967, and 7, (1), June 1968. One point brought out in this 
bibliography is the frequent presence of nitrates in artifical patinas not commonly found 
I in natural patinas. 

i T. Weisser: Is there a difference between the natural and artifically formed patinas with 
respect to the adhesion of the corrosion products, the depth of the corrosion, the crystal 
size or crystal formation in the patina, since one is formed slowly and the other quickly? 

J. Kruger: Morphology can be an important element in determining age even though there 
are ways to promote grain growth. Hemming showed SEM's of the surface structure of an 
' artifical patina and that of a naturally formed patina. The morphologies were strikingly 
different, the artificial one being looser and the natural one being much more close-knit. 

L. Fitzgerald: Concerning the artificial patina (INCRApatine II) morphology compared with 
natural patina, the SEM photos shown of the INCRApatine II were newly grown, fresh crystals. 
The natural patina had been exposed to the weather for ten years and the crystals were 
understandably beaten down and compacted. 

1 B. Viedhota: Can we use the common mineral term for a corrosion product when it was not 
formed in the context of natural geological processes, independent of human manipulation? 

C. S. Smith: Give the mineral name so as to identify the crystal structure and the chemical 
composition, as an aid to our memories. 

R. M. Organ: The chemical composition can be the same for several minerals. 

W. T. Chase: Is the abundance ratio of atacamite to paratacamite indicative of the 
authenticity of a patina; atacamite being indicative of a forgery? (See S. Z. Lewin, A 
new approach to establishing the authenticity of patinas on copper-base artifacts, in 
:i Applications of Science in Examination of Works of Art, William J. Young, ed., Boston 
[ 1973; and J. B. Sharkey and S. Z. Lewin, Conditions governing the formation of atacamite 
j and paratacamite, American Mineralogist , 5£ (1-2) 179-192 (1971).) 

W. D. Richey: I have had difficulty in reproducing the work--which may tell you more 
about my work than about Lewin and Sharkey. Both mineral forms were apparently present on 
i objects that seem naturally patinated. It is not as easy to distinguish the two minerals 
as those articles seem to suggest. 

E. V. Sayre and L. van Zelst: Because of this question of atacamite/paratacamite has been 
raised regarding the Greek bronze horse at the Metropolitan Museum, Kate Lefferts, Larry 
j Majewski, Pieter Meyers, and I have been trying to gather information on the frequency with 



209 



which particular abundances of atacamite/paratacamite have been observed on ancient bronzes 
which are thought to be above reproach. Most laboratories have reported both minerals being 
present. Tony Werner reported in a personal communication that the majority of the authentic 
bronzes he had examined at the British Museum had a combination of both atacamite and 
paratacamite. Pieter Meyers and Bert van Zelest report that more often than not on the 
many bronzes they have examined at the Metropolitan Museum there is a combination of both. 
Flossie Whitmore and Bill Young at the Museum of Fine Arts, Boston, have observed the 
same. Direct conservation observation has not proved the presence of atacamite to be a 
reliable indicator of forgery. One might ask why. 

What conditions would produce a mixture of both minerals? In Lewin's first paper, it 
was stated that paratacamite could be produced under conditions where there were small 
concentrations of copper complex ions present, but atacamite only when there was an appre- 
ciable concentration of copper complex ions. This, therefore, raises the question of the 
basic solubility parameters for these two substances. I believe literature exists only on 
the solubility product of paratacamite obtained from precipitation measurements. If you 
calculate using this solubility product for any system where there is much buffering, as 
certainly occurs under soil conditions, you find that atacamite and partacamite should be 
fairly soluble materials, surprisingly as most conservators tend to think of them as being 
very insoluble. We indeed have found this to be the case. Bert van Zelst has taken over a 
research at the Metropolitan Museum in which Pieter Meyers and I have tried to determine 
solubility products for both minerals in various buffered solutions. In preliminary studies 
we found that in something like an acetate buffer, with a pH of 5, you get bright blue 
solutions, with a corresponding high complex copper ion concentration. Even when you have 
fairly basic conditions, adding some sesquicarbonate (with a pH of 11) to pure paratacamite, 
you get a distinct blue solution. 

R. M. Organ: I believe the first paper on this subject was done by Helmut Otto (Freiberger 
Forschungshefte B37, 56-77 (1959)), who made x-ray diffraction patterns and thought this 
was a clear way of distinguishing between ancient and modern patination. 

E. V. Sayre then gave a demonstration prepared by L. van Zelst: As a demonstration a 
freshly prepared mixture of pure paratacamite with a 5 percent aqueous solution of sodium 
sesquicarbonate was compared with a similar mixture which had been combined some two weeks 
earlier, and with a mixture of pure paratacamite with water. It was observed that the 
freshly prepared solution of paratacamite in aqueous sodium sesquicarbonate had a quite 
pronounced blue color, that the identical but aged solution has a much less intense blue hue, 
and that the mixture of paratacamite and water was nearly colorless. It was also observed 
that the residual solid in the aged sesquicarbonate-paratacami te solution was more granular 
and more blue in color than that in the other two solutions. It has been found through x-ray 
diffraction that upon aging the paratacamite in the sesquicarbonate solution was slowly 
changing to malachite. 

From these observations the following could be inferred: 

(1) That, contrary to much accepted opinion, paratacamite itself is appreciably soluble 
in the normally employed sodium sesquicarbonate treatment solution. Nantokite, of course, 
also dissolves in this solution. 

2) That not only is chloride removed in part by direct solution of paratacamite, but by 
direct conversion of the paratacamite to malachite. Nantokite is not required for this 
conversion. 

(3) The fact that the concentration of the cupric complex ions in solution responsible for 
the blue color first reaches a maximum and then quite slowly decreases with time indicates 
that the physical chemical process, e.g., complex ion formation, precipitation, etc., which 
are bringing the system to equilibrium are occurring so slowly that there is time for many 
processes affecting the final nature of a corrosion layer, e.g., corrosion itself and the 
diffusion of ions, to take place under nonequilibrium conditions. 

Pieter Meyers, Lambertus van Zelst, and Edward Sayre are in the process of making 
quantitative studies of the solubility parameters and rates of change of various corrosion 
products of copper alloys in buffered solutions. 



210 



It has been observed that paratacamite has the same sort of inverse solubility that 
calcite does, it becomes less soluble by a factor of 10 from that reported in the 

literature. There has been little success approaching stable conditions from the super- 
saturated side so as to establish that equilibrium has indeed been obtained. There has been 
difficulty in getting material to precipitate out of the saturated solution. 

They have found the solubility of atacamite and paratacamite in some buffered solutions 
of pH closer to that of normal soil than sodium sesquicarbonate has been as great as a few 
tenths of a gram per liter. Thus, the basic premise of Lewin's conclusion that atacamite 
should not appear on naturally corroded bronzes because such natural corrosion would 
normally take place under conditions in which the concentration of complex cupric ions was 
extremely low appears to have been incorrect. There is good reason to expect appreciable 
amounts of copper-containing ions to be present in solution during natural corrosion, and 
hence, that both atacamite and paratacamite will be formed as has been found to be true on 
many ancient bronze objects. 



R M Organ: The sesquicarbonate treatment for "bronze disease" is meant to be used only 
tor the CuCl-Cu20-copper carbonates (azurite-malachite) systems, NOT for a basic cupric 
chloride system, which can be a green patina in its own right, with paratacamite not 
evidence of bronze disease. "Bronze disease" consists of the paratacamite on or in a 
cuprous oxide crust, appearing and developing, spot to area. Perhaps Ed Sayre could 
consider the place of azurite in his research, because we do observe that malachite greens 
sometimes change toward blue (azurite) in sesquicarbonate solution. 

S. Reisman: Is there a stratigraphy to the artificial patination of copper, or is the 
copper sulfate formed directly on top of the metal? 

L. Fitzgerald: In almost any kind of patination system there is an oxide structure from 
cupric to cuprous to metal. I know of no system without a microlayer; you just don't drop 
two electrons that quickly. The layer may only be one molecule thick, but it will be there 



Question 6: How satisfactorily can the history of an object be inferred from its corrosion 
products? 

C. J. Jack: In the before-cleaning view of the Graeco-Roman silver wreath shown in 
figure Q-6-1, there is an appearance of lamination (not visible in the photograph). We 
have no knowledge of its age or origin. An analysis by Dr. Ursula Franklin found silver 
and copper sulphide on the wreath before cleaning. Upon cleaning (fig. Q-6-2) the analysis 
showed the leaves to be a silver copper alloy of the same composition as Roman coinage of 
about 400 A.D. A number of treatments were tried to clean the wreath: (a) 30 percent hot 
formic acid; (b) Goddards silver dip; and (c) electrolytic reduction, ultrasonic cleaning, 
silver dip, quick drying in alcohol and coating with Incralac. The last treatment (c) was 
the most satisfactory one. It removed all of the two outside laminated layers, leaving 
only the inner silver copper alloy. 

The question posed by this work is, could the laminating layers be produced in the 
laboratory, or would it take exposure going back to 400 A.D? 

W. T. Chase: A number of the objects from the Man-ch'eng tombs of the Former Han Dynasty 
(see Question 1 for sources) show different patinations. These include the shiny black 
mirrors, a matte black censer, a gilded lamp with green spots, and some objects with 
reddish corrosion which looks like cuprite. These all came from the same, rather dry 
tomb, apparently without earth fill, but they have different corrosion products. Why? 
How can we relate these corrosion products back to burial conditions? 

E. Esaalante: An important factor is the nature of the environment. Our work on metals 
buried in soils indicates that one can produce layers of many different thicknesses, not 
only as a function of time, but as a function of the nature of the environment. Therefore, 
the nature or amount of the corrosion product cannot be used as a reliable guide to judge 
the age of an object. 



211 



Figure Q-6-1 . Wreath of Bay leaves. Graeco-Roman , 
silver with a small amount of gold foil. Front 
view before cleaning. 




Figure Q-6-2. Wreath of Bay leaves. Graeco-Roman, 
silver with a small amount of gold foil. Front 
view after cleaning. 



Van Zelst: A question of important practical value for the museum laboratory is in how 
far corrosion structures are indicative for authenticity. Especially, the phenomenon of 
intergranular corrosion is often regarded as a sign that shows natural corrosion processes 
have taken place. Thus, does everybody think about the usefulness of the observation of 
intergranular corrosion as an argument in favor of austenticity? 

Kenneth Morris: We have seen intergranular cracking in parts of one artifact thought to 
be ancient when another part of the same object thought to be a new repair did not show 
cracking. Both parts showed similar outer corrosion products and were from adjacent areas 
of the same object. 

C. s. Smith: Someone ought to do some experiments on the important question as to whether 
intergranular corrosion indicates long exposure. It seems reasonable, for example, that 
corrosion at elevated temperatures in an autoclave at 150 °C or so would produce effects 
that are similar to long time corrosion including diffusion down grain boundaries. 

M. Goodway: The metallurgical literature contains references to stainless steel instruments 
having failed by intergranular corrosion in autoclaving. 



212 



T. D. Weisser: I have been able to produce intergranular corrosion in brass with sodium 
hydroxide. 

J. Kruger: Corrosion engineers see failures by intergranular corrosion in many situations 
when the time of exposure is not very long when compared to that undergone by ancient 
metal artifacts. If anyone wants intergranular corrosion, I can produce it on order for a 
number of different alloys. 



Question 7: What is the mechanism of the preservation of an original surface in heavily 
corroded bronzes and silvers? 

C. S. Smith: I suspect that the retention of the topographic details of the original 
surfaces in many corroded bronze objects is a result of the interaction of two factors. 
Corrosion of most copper alloys begins by deep penetration along grain boundaries and (in 
castings) along the course of dendrite cores, and thereafter it proceeds laterally under 
conditions of rigid mechanical and volumetric restraint. Since the mineral corrosion 
products have a volume greater than the metal from which they form, some of the positive 
ions must migrate by diffusion to the outside of the piece, where they precipitate without 
restraint in a softer, loosely packed, less coherent form. Mineralogists have long known 
that the replacement of one mineral by another (for example, zinc sulphide, ZnS, replacing 
pyrite, FeS) occurs volumetrically, not in the amounts called for by chemical equivalents. 
The vani shingly thin but continuous layer of liquid maintained by capillarity between the 
old and the new phases is enough to allow the unwanted cations to diffuse out and oxygen 
or other anions to diffuse in. The slow rate of growth and the mechanical restraint 
combine to give a dense mineral product. The outer crust of the corroded object forms 
under quite different conditions away from direct contact with the metal, and the two 
zones are usually easy to separate. Iron, in which intergranular penetration rarely 
occurs, does not preserve surface details. It would be interesting to see what happens 
with a homogeneous single crystal of alpha bronze, or with an amorphous alloy. 

The corrosion of metals over archaeological time periods is more akin to mineral 
formation than it is to the industrial corrosion mechanisms with which scientists and 
engineers today are mainly concerned. The next interdisciplinary conference on corrosion 
and conservation of archaeological objects should include a petrologist interested in ore 
formation. We could even tell him something, for we at least know the composition and 
structure at one earlier stage of history. 



Question 8: What is the mechanism of the pseudomorphic replacement of the bronze structure 
with stannic oxide? 

W. T. Chase: Figure Q-8-1 shows a Shang Dynasty Chinese broad-axe which we recently 
restored at the Freer Gallery. During the restoration we were able to take a number of 
cross-sections. Figure Q-B-IA (also see color plate e) shows one of these cross-sections. 
The tin oxide on the outside of the axe preserves the dendritic structure of the casting. 
A detail of the inner transition zone at greater magnification (fig. Q-8-2) shows uncorroded 
remnants of alpha dendrites, with what looks like cuprite filling in between them. Just 
outside of this is a black corrosion product, with the dendritic structure preserved in 
what looks like a diffusion banding of some sort. I do not really have an understanding 
at all of what is going on here. It would be very helpful if someone would work up a 
computer simulation of what is going on in terms of the various ions and molecular species 
in a corroding bronze like this one. 

K. Z. Holm: I cannot answer the question but would like to point out, that according to 
my experience, one never finds the cuprous chloride together with this form of pseudomorphic 
mineralization. You can find the characteristic formation of warts connected with the 
chloride-induced corrosion in some parts of an object and, in other parts of the same 
object, the pseudomorphic change we talk about here. 



213 



Figure Q-8-1 . Broad-axe of the 
type ah'i in the collection 
of the Freer Gallery of Art 
(FGA 46.5). The axe is 12 7/8 
inches long (32.7 cm). It 
was recently repaired by the 
Technical Laboratory of the 
Freer Gallery. The length of 
the tang was increased, with 
the intention of correcting 
previous erroneous restorations. 
The restored area can be seen 
just below the animal mask. 



■t 




Figure Q-8-1 A. Cross-section from the 
haft of ch'i 46.5 (see fig. Q-8-1 and 
color plate e). The section was 
removed during restoration. The outer 
surface of the haft is at the top. 
White tin oxide can be seen to have 
psuedomorphi cal ly replaced the orig- 
inal dendritic structure. Below this 
is a banded red layer, probably con- 
taining cuprite, and below this is a 
black transition zone, where the den- 
dritic structure is preserved. The 
dentrites and their coreing are seen 
to carry through from the metal into 
the black material. At the left side 
is an artificially ground edge made 
by the previous restorer. Dark field, 
polarized, magnification ca. XI 00. 



Figure Q-8-2. A detail of the black material on the 
inside of dh'i 46.5. At the center of the photograph 
can be seen remnant metal and an orange (light) mater- 
ial, which may be cuprite. Above this is the black 
material which psuedomorphi cal ly replaces the original 
dendrites. Dark field, unetched, magnification X200. 



Question 9: What are the critical temperature and relative humidity for the conversion 
of cuprous chloride to paratacamite? 



and 



Question 11: What are the environmental conditions favoring the formation of paratacamite 
over atacamite or viae versa? 

M. Pourbaix: The conversion of cuprous chloride to paratacamite is not only a question 
of relative humidity and temperature. Oxygen concentration or partial pressure is also 
important. Cuprous chloride exists only in the absence of oxygen at a low pH (see fig. 
Q-9-1). Paratacamite, on the other hand, needs oxygen in order to exist. This then is an 
important difference in the set of conditions that determine which substance is stable. A 
diagram such as that shown in figure Q-9-1 allows one to predict the stability of parata- 
camite. On this diagram are the partial pressures of water represented as different 
lines. The slope of these lines on a plot of 1/T vs. log p02 (T = temperature K, and p02 
is the oxygen partial pressure) is a measure of the enthalpy of condensation of water 
vapor. If you have a substance which is hygroscopic for one atmosphere, there is an 
increase of the boiling temperature and a decrease in the partial pressure. Another line 
can be drawn that represents the partial pressure of the saturated solutions of the compounds 
shown in the diagram. On this diagram one can point out regions of stability for these 



215 



+1 



CM 
O 
Q. 

C3 
O 



-3 - 







RELATIVE 






HUMIDITY 


- 




100% 






56% 


- 


PURE WATER 


^ 33% 






y^ \ SATURATED 






/ SOLUTIONS 




^^^^^ / A T A r* A nyi 1 Tc 
/ A 1 AO AM Mb 


dU I ALLAUNl 1 b 




PARATACAMITE 




r I 


1 1 1 1 1 1 1 1 ■■ 1 1 


■ ' 1 ' ' ' ' 1 X 10° 




273 298 


373 T 



T (K) 



Figure Q-9-1. Stability conditions for CuCl2- 3CuO-nH20 in the presence 
of water vapor. 

compounds. To determine the condition, environmental conditions favoring the formation of 
paratacamite over atacamite or vioe versa, one can use the enthalpies of formation of 
these compounds which can be determined on a diagram such as figure Q-9-1 which shows 
regions where these compounds may be formed. All one needs is to have good values for the 
thermodynamic properties, i.e., free enthalpies and enthalpies of formation, of paratacamite, 
atacamite and other corrosion products of interest. 

R. M. Organ: I did a very crude experiment in 1960 where I put a small amount of cuprous 
chloride on a watch glass and exposed it to steadily increasing relative humidities and 
found that below about 39 percent relative humidity it did not change. Above 39 percent, 
it went to green paratacamite. 



Question 10: For the stabilization of pockets of cuprous chloride on bronzes from the 
Near East is anything better than control of relative humidity to below 
40 percent? 

and 



Question 12: What are the criteria to judge the success of treatment of bronze disease? 



C. S. Smith: Though 
the corrosion of chl 
should be considered 
degrees centigrade a 
slow rates. Change 
UNESCO auspices stor 
cave in the Antarcti 
political stability, 
I bel ieve Mr. Organ 



control of relative humidity is an established procedure for stabilizing 
oride-containing bronzes on display, control of temperature also 

for long time storage of research materials. Below about minus ten 
Imost all relevant biological and chemical processes proceed at extremely 
is virtually prevented. One might envision a kind of Shoshoin under 
ing a fraction of mankind's most valued objects and records in an ice 
c. This would be self-preserving in the event of a breakdown of 
and could have high ceremonial access only once a century, otherwise, 
has studied low-temperature storage and I hope he will comment. 



216 



R. M. Organ: In order to control corrosive attack on an immense number of objects, such 
as IS found in Smithsonian museums, numbering between 15 million and 15 billion, control 
of environment, such as very low temperature or very low relative humidity (for metals) 
IS the only practicable method--individual attention to so many objects is impraticable' 
Storage in the Antarctic could be ideal! 

B. F. Brown: Is it not possible to use a surface active compound {e.g., SPRA-DRI or CRC) 
to displace the moist chloride from the metallic surface? Such materials have been useful 
in preserving metal surfaces which have been in sea water without the benefit of rinsing. 

C. E. s. Hett: In parks in Canada, we have experience with the watering fluids; these are 
commencal products made by Sunbeam Auto-Corrosives in Toronto. There are two classes 
one water miscible and the other not water miscible. In all, about six or eight types'are 
produced under the trade name of Ferromedes. The non-water miscible types have the advantage 
Of an oil or wax in the solvent and are designed to hold corrosion of the iron after air 
drying until laquering. We use the Ferromedes both on cast and wrought iron artifacts 
after boiling chloride free and have had satisfactory results so far in stabilizing when 
this treatment is followed by a phosphating lacquer, produced by the same company. 

J. Kruger: I would like to enter into the proceedings some other information bearing on 
the treatment of bronze disease that I recently received from Dr. U. R. Evans of Cambridge 
University, who is considered by many to be the founder of corrosion science. The letter 
goes as follows: 



Dear Dr. Kruger: 

I am most interested in the dialogue between Museum Conservators and 
Corrosion Scientists which you, with my good friend Prof. Pourbaix and others, 
are organizing for March 17, and wish it every success. 

Had there been more time, I would have written to you to inquire whether 
a paper from someone who could not attend in person would be welcomed. Had your 
reply been "yes", I would then have suggested to Mr. Rayner of the Fitzwilliam 
Museum, Cambridge, that he should send you an account of his success in dealing 
with a virulent outbreak of bronze disease 25 years ago. The 500 specimens 
treated in 1951 are today in good condition, and I think that we may assume 
that the cure is permanent . 

Unfortunately, Mr. Rayner has been on leave and only returned today. 
Clearly it is too late for a paper, but he has produced a short statement 
of his procedure , which I enclose, along with xerox copies of the four accounts 
on the situation, printed in my books of 1960, 1968 and 1976 and in a lecture which 
I delivered in 1951^. If you are interested and have time, you might care to look 
through this and convey your impressions to the meeting. I do not press this; you 
may have no time, or it may be against the rules governing the meeting. 

The circumstances were these. At the outbreak of the second world war in 1939, 
the most valuable bronze antiques in the museum were sent to a place in the West 
of England considered to be relatively safe from enemy bombing. After the war 
was over, they returned, but apparently owing to acetic acid vapor picked up from 
wood shavings in the packing cases, local corrosion broke out, sometimes leading 
to perforation in a matter of weeks . There was a serious risk that these exceptually 
beautiful and extremely valuable objects of art would be utterly ruined. We had to 



■U. R. Evans, Chemistry and Industry , April 25, 1951, p. 710-711. 

U. R. Evans, The Corrosion and Oxidation of Metals (Edward Arnold Publishers Ltd., 

London, 1960) p. 402. 
U. R. Evans, Ibid., First Supplementary Volume, 1968, p. 373. 
U. R. Evans, Ibid., Second Supplementary Volume, 1976, p. 274. 



217 



move quickly, and I worked out roughly a method based on the application of HCl , 
HsPOij and NaaCOs, in turn, on a zinc nib. Rayner put this into practical form and 
treated 500 specimens without decay. The treatment worked will, and the acetate 
anions were fetched out of the corrosion pits under the E.M.F. Zn/bronze. Rayner 
has written that it is "far the most effective treatment that I know and also the 
simplest. " 

With our wishes for success of the meeting and greetings to my friends, in- 
cluding Dr. Floyd Brown. 

Yours sincerely. 



Ulick R. Evans 



N. c. Rayner: Treatment of Ancient Bronze Artifacts Suffering from "Bronze Disease." Cut 
out a "nib" from sheet zinc and join this to the negative terminal of a galvanometer. 
Scrape the specimen at some unobtrusive point until metallic lustre appears (do not confuse 
red cuprous oxide with the true metal) and connect this point to the positive terminal. 
(See fig. Q-12-1.) Wet the nib with dilute hydroohloric acid (2 parts to 5 parts) and rub 
the disease spot until the green material is removed. As far as possible, confine the 
wetting to the spot (or spots) to be treated, if necessary, grease the surrounding area 
with vaseline and rub away the vaseline with the nib at the point to be treated. As soon 
as the green powder is destroyed, remove the hydrochloric acid on blotting paper, then 
apply syrupy phosphoric acid with the nib and blot off. Finally apply sodium carbonate 
with the nib and blot off. Treatment is then complete. 




igure Q-12-1. Sketch of system for treating ancient bronze 
artifacts for "bronze disease." 



218 



F. M. Organ: I have always had the impression from some of the published accounts of 
Rayner's treatment that the zinc nib was simply touched to the affected spots without any 
electrical connection to the metal, as shown in the sketch on page 218. This information 
that an electrochemical cell must be made between the zinc and the bronze is thus quite a 
valuable contribution. One hopes that the treatment is permanent because the treatment of 
individual spots on a large object can be quite a lengthy process with four or five spots 
per square centimeter under the best of conditions. 

W. T. Chase: The following three figures show a section from a Thai bronze of the Ayudhya 
Period (ca. 300 years old) which contains a thick cuprous chloride layer. These photo- 
micrographs were taken with the intent of showing the rapid progress of bronze disease; 
they appear in color in a catalogue entitled Bronze Disease and its Treatment which I 
wrote for a joint project on treatment of bronzes in Thailand. The catalogue is available 
from the Thai National Museum, Bangkok, and was printed in 1975. Figure Q-12-2 shows the 
section after exposure to 100 percent relative humidity for 15 minutes; no bronze disease 
can be seen. After two hours, one can begin to see a change in the cuprous chloride layer 
(fig. Q-12-3). The change can be seen to begin within the cuprous chloride layer, and not 
at the interface between the bronze and the cuprous chloride. After four hours, the 
transformation of the cuprous chloride is more pronounced, (fig. Q-12-4), and it continues 
to grow on further exposure to high humidity. 

The main thing to note from these figures is that the green spots which are the 
outward manifestations of bronze disease have nothing to do with the corrosion of the 
metal, but are simply due to the change (oxidation and hydration) of the cuprous chloride 
layer. All of the criteria for judging success in the treatment of bronze disease depend 
on the green spots not reappearing after treatment. These figures serve to point out that 
these green spots have nothing to do with the corrosion of the underlying metal. Thus, the 
usual criteria for success are not really satisfactory, although I continue to use them. 

In the case of the benzotriazole treatment, when a bronze has been treated with 
benzotriazole and sectioned, the bright green can be seen to break out again in the cuprous 
chloride layer on exposure to high humidity. It may be that benzotriazole only penetrates 
to the outer surface of the cuprous chloride layer, and not through it. If it does this, 
it may be stopping the transformation of the cuprous chloride to cuprous oxide; this would 
mean that the Cl~ ions are not freed to react again with the copper metal. If benzotriazole 
does indeed work in this way, it may really be a successful treatment for bronze disease. 
It would be nice to have some experimental corroboration for this hypothesis. 

M. Pourhaix: There was, as you know, a meeting at Faerrara on September 15, 1975. There 

were several questions raised concerning the protection of bronzes and gilt bronzes with 

benzotriazole. Something that is presently not known is what is the mode of action of the 
benzotriazole and of other inhibitors with copper. 

I had up till now no opportunity to work on the action of benzotriazole on copper and 
on copper alloys. The opinion I presently have on this subject is the following one: it 
is probably not true that benzotriazole, as has been very often said, is really working 
by simple adsorption. I believe that its action may be merely a chemical action on the 
cuprous oxide, which very often exists on the surface of the metal. We know, and you 
know, that cuprous oxide exists on corroded copper and bronzes as both an adherent and 
somewhat protective coating, and a nonadherent and loose nonprotecti ve sludge, formed as 
an hydrolysis product of cuprous chloride or of dissolved copper ions. I have the feeling 
that benzotriazole is efficient when CU2O exists as an adherent coating, and that it is 
not efficient when CU2O exists as a sludge. This concept, which might be rather easily 
checked, might perhaps help to clarify, as has just been said by Tom, the method of action 
of benzotriazole and of similar compounds; this might help to elucidate the conditions where 
these compounds may be successfully used for the protection of copper and of copper alloys. 

R. M. Organ: While the cuprous chloride changes into green paratacamite, i.e., the onset 
of bronze disease, the cure of the disease is also taking place because the production of 
paratacamite is destroying the cuprous chloride. When all the cuprous chloride goes away 
by converting to paratacamite, it cannot react anymore and bronze disease is stopped. 
' What is not stopped, however, is the continuing corrosion of the metal under the crusts of 



219 



Figure Q-12-2. Photomicrograph of 
sample containing a thick cuprous 
chloride layer after exposure to 
100 percent relative humidity for 
15 minutes. No bronze disease 
can be seen. 




Figure Q-12-3. After two hours a 
change can be seen in the cuprous 
chloride layer, not at the inter- 
face between the bronze and the 
cuprous chloride. 




Figure Q-12-4. After four hours 
the transformation is more 
pronounced. 



cuprous chloride. That corrosion can only be prevented by benzotriazole or by controlling 
the relative humidity to a value so low that the cuprous chloride cannot react. Cuprous 
chloride does not convert to paratacamite below a relative humidity of 39 percent, but 



220 



what is not so well known is the relative humidity below which the continuing penetration 
of cuprous chloride will be halted. This penetration is the real danger in the museum. 
It is because bronze disease stops this penetration that I have made the heretical state- 
ment that bronze disease is good. 

T. D. Weisser: Isn't hydrochloric acid a by-product of the conversion of cuprous chloride 
to other products? If so, can't this hydrochloric acid react with the remaining copper to 
form more cuprous chloride? 

R. M. Organ: If you do, I would guess that it vaporizes into the air. It is a valid 
point that should be looked at. 

L. Barkman: If one takes identical sesquicarbonate solutions and adds identical quantitites 
of paratacamite to them, it can be seen that such a combination prepared now is considerably 
more blue than the similar solution prepared a week ago. The conversion to malachite has 
proceeded much further in the latter solution. The point is that we are dealing with slow 
processes and that it is to be expected that there will be intermediate stages where non- 
equilibrium conditions prevail. Therefore, to understand the chemistry of processes 
involved in bronze disease, it is important to take rate considerations into account. 

M. Pourbaix: If one assumes that bronze disease continues only in the presence of a humid 
environment, then a consideration of the hygroscopic nature of the solid compounds formed 
on the surface is of utmost importance because it influences the critical relative humidity 
above which the surface becomes wet. One can get some idea of the hygroscopic nature of 
paratacamite, for example, by measuring the vapor pressure of a saturated solution of it 
at various temperatures. If one then keeps the relative humidity below these values for 
the vapor pressure, the solid will dry out and bronze disease is stopped. 



Question 13: What are the prospects for the hydrogen treatment of corroded bronze containing 
cuprous chloride at less than 300 °C? 

E. Birahenall: A valuable reference that would help with answering this question is a 
paper by H. H. Kellogg, Trans. AIME 188 , 862-872 (1950). The compilation, Thermodynamia 
Tables for Process Metallurgists, by C. Jbrgensen and I. Thorngren, published by Almquist 
and Wiksell, Stockholm, 1969, also contains useful tables of free energies and enthalpies 
of chlorides as well as oxides, sulfides and other compounds that one might be tempted to 
reduce with hydrogen. 

D. Pieohota: I thought it was possible to sublimate iron chloride from iron at around 300 
degrees or so centigrade. In an appropriate environment you might not oxidize or reduce 
the oxide layer, but you could merely sublimate out the chloride. Is that not so? 

R. M. Organ: I think that was the annealing process, so called, at a temperature of 800 °C 
described very fully in Eriksen, E. and Thegel , S., Conservation of Iron Recovered from 
the Sea, Copenhagen, 1966, 29 to 41. 

W. T. Chase: What can be done in the way of changing the environment to stabilize lead 
objects until they can be treated? 

R. M. Organ: If the excavator can carry along Permalife envelopes, he can just pop the 
leaden objects into them. The Permalife paper contains calcium carbonate which will 
absorb any volatile organic acid on its way to the lead coin. Once organic acids reach 
the lead, they stimulate corrosion and rapid decay sets in. Once the lead is in the 
laboratory we can regenerate lead from any corroded material in the exact position it is 
in now. The other thing you can do in a hurry, if the lead is already corroded, is to put 
shellac varnish on it. That would hold everything in position until it can be treated and 
you could still read any inscription. 



221 



M. Goodway: Since the lead corrosion problem (in wooden drawers) was first observed in 
the British Museum, how do you get that information back to the archaeologists rather than 
just to the conservators? 

R. M. Organ: That's a problem. In the past there has been little interaction between 
conservators and archaeologists. That is the reason why at the Smithsonian we have lectures 
on conservators' techniques on videotape (available for archeologists to view). There are 
some books that suggest things to do, such as one by Dowman, but they won't do for a site 
such as the one Mr. Trousdale described. 



Question 14: Are there any differences between bronze disease caused by chloride and that 
caused by acetate? (No discussion) 



Question 15: What are the cleaning, stabilization and storage procedures applicable to 500 
early iron age fragments of bronze from a Hallstatt culture site? These have 
a discontinuous, unoxidized metal covered by a vile powder layer and then a 
noble chipable layer. 



P. V. Pieahota: These slides are views of selected pieces of the Peabody Museum's Hallstatt 
period bronze collection known as the Duchess of Mecklenberg Collection. The pieces come 
from the Weinitz cemetery in Austria, excavated in 1907 to 1917. Mr. Gettens analyzed the 
corrosion layers in 1936, finding that there is commonly a fine but crumbling tin oxide 
layer which covers a troublesome powdery blue layer. He found that this layer is similar 
to a rare tin silicate called Arandisite. He found no chloride, sulfate or carbonate on 
the pieces with the above corrosion products. There has been no treatment of these pieces 
apart from superficial dirt-removal and repair. How have these layers formed? What is 
the state of the art of the plastics and waxes usable for consolidation of these pieces? 
Is it possible that there is still a corrosion process going on? 

E. M. Organ: The only case in which I have seen silicates on bronze was in the Institute 
of Archeology in London--it was treated satisfactorily in fused sodium carbonate mixture^. 
We don't have any consol idative reduction techniques for copper corrosion products such as 
you would need for these. Is there any evidence of disease apart from cracking off of the 
crusts? 

D. V. Pieahota: Yes, on some pieces where we have chloride but doesn't show the tin oxide 
layer. I feel it is stable, the major problem is consolidation. 

R. M. Organ: I think what I would do is to get the pieces thoroughly dry in a desiccated 
atmosphere and then consolidate them with a microcrystal 1 i ne wax. Take care that everything 
unwanted is taken off the surface. Don't try it on any object too large to go into a 
Soxhlet extractor because someone in the distant future will wish to remove the wax. 

D. V. Pieahota: Our museum is dirty and we wouldn't want dirt in the wax. 

R. M. Organ: In order to be sure of ability to remove a resin coating, one would want to 
choose one which has a proven ability to be removed, probably an approved polyacryl ate, 
one which will not cross-link with oxygen in time. I think by far the best prospect is 
afforded by a microcrystal! ine wax on several counts: it is consol idative coating, it is 
always removable, and it has minimal permeability to moisture so that if your dry atmosphere 
gets out of control there is still some protection. To control the problem of getting 
dust on it you'd have to choose a hard wax. 

D. V. Pieahota: We went to a softer wax because of brittleness in hard wax. 



^Cornwall, I. W., and Gedge, I., A new Method of Cleaning Corroded Bronzes, the Annual 
Report of the Institute of Archaeology, London Univ., 10, 34-36 (1954). 



222 



R Organ: Some people have handled the dirt problem for storage drawers by putting a 
Plexiglas cover on each drawer so that one could see the objects without opening up the 
storage space and fingering the objects. 

W. T. Chase: (1) Gloss on a resin-covered surface can be avoided by drying thermoplastic 
impregnated pieces over the impregnating tank in a solvent atmosphere. (2) To detect 
remnant metal in objects of this sort, a miniaturized commercial metal detector (see 
Question 23) can be used. We use a modified Radio Shack kit to detect remnant metal. Is 
there any metal left in your objects or are they completely mineralized? 

D. V. Pieohota: No, they are not completely mineralized; in many objects there is a 
continuous metallic core. 



Question 16: What is the effect of benzotriazole on the mineral substances of corrosion 
crusts on bronze? 



W. D. Riohey: BTA interacts with Cu(II), not Cu(I), in general, when it interacts with a 
patina. (The exception would be CuCl , nantokite, if the treatment solution reaches that 
level.) Since Cu(II) BTA compounds are relatively soluble (relative to patina materials), 
it is probably scavenging free Cu++ in solution in pores. BTA does not seem to modify 
CU2O, and only to a modest extent attacks malachite. The relatively soluble copper basic 
chloride converts to some extent. Cu(I)Cl undergoes a surface reaction but does not 
completely convert pieces or particles, which may be a good thing if you consider the 
relatively low density of Cu(I)BTA. 



Question 17: Are there any known adverse effects between fixed and vapor phase inhibitors? 
and 

Question 18: What is the state of the art of vapor phase inhibitors applicable to museum 
use, and are any of them potentially hazardous to health? 



S. Cobum: Vapor phase inhibitors (VPI) such as those developed by Shell orginally were 
organic nitrites. Their use are of wide interest. The material functions in a manner 
similar to "moth balls." A powder is placed in an enclosed space. This powder vaporizes 
and deposits on the active sites on the surface a protective film. Two references are 
very useful; Vapor Phase Inhibition of Atmospheric Corrosion, Sexton, Corrosion Technology, 
May 1960, and The Distribution of a Volatile Corrosion Inhibitor on Corroded Iron, 
Henriksen, Corrosion Soi. 12_, 433 (1972). 

B. A. Miksia: The inhibitor that Mr. Coburn mentioned that was made by Shell is 
dicyclohexyl -ammonium nitrite (DICHAN). It is used primarily for steel and, of interest 
to museum conservators, is the fact that it is not compatible with non-ferrous metals as 
j such. In fact, nitrites can have adverse effects on non-ferrous metals as brasses, 
. copper and bronzes under conditions of high humidity and temperature. We have been 
] concerned with developing inhibitors that provide corrosion protection to both ferrous 
and non-ferrous metals. Our experience indicates that the benzoates would be very promising 
for museum application, providing 10 to 20 years of protection. It has been used in 
Norway for 15 to 20 years without any problems where carbonates were also used. Humidity, 
pressure of SO2, H2S at reasonable concentrations does not affect their ability to protect 
metals. The benzoates are effective in inhibiting corrosion by chlorides, although the 
nitrites are more effective against chlorides. 

Our research has shown that these inhibitors work by adsorbing as monomolecular 
layers on less than 1/2 percent of the surface with the sites of adsorption changing from 
moment to moment in a statistical fashion. 



223 



With regard to the second part of the question concerned with toxic effects, our 
research has indicated that toxicity of vapor phase inhibitors varies between 200 to 2000 
mg/kg of body weight depending upon their composition. It was found that benzoates are 
less toxic than nitrites which are presently in use. The question on toxicity of volatile 
corrosion inhibitors is a very complex one. Generally, it is believed that compounds derived 
from carboxylic and carbonic acids are less toxic than salts of nitrous and chromic acids. 
Nitrites are of primary concern due to the possibility of forming nitrosoamines in the 
human body. 

M. Pourbaix: We think that the action of vapor phase inhibitors is exerted by dissolution 
in the water present on humid articles, so that the efficiency of these inhibitors may be 
correlated with their inhibitive action on this condensed water. If so, the eventual 
adverse effect of polluting gases, such as SO2, H2S, or CI 2, might be evaluated by experiment 
performed with aerated aqueous solutions of these gases. Can Dr. Miksic comment on this? 

B. A. Miksia: In answer to Prof. Pourbaix' s question regarding the solubility of VPI's, I 
must say that to the best of my knowledge the functional relationship between water solubility 
and the effectiveness has not yet been established. There have been some indications in 
the literature that such a relationship does exist, but I believe that the major factor 
which contributes to the effectiveness of VPI's in a specific environment is their protective 
mechanism. For an illustration, it was discovered that VPI's which produce hydrophobic 
(water repelling) effects often fail in salt atmospheres, probably due to higher conductivity 
of corroding electrolyte (chloride contaminated water). On the other hand, VPI's whose 
protective mechanism is based on changing the electrochemical potential of metal substrate 
have been found much more successful in inhibiting corrosion. Probably the most effective 
inhibitors are those formulations that combine both above mentioned mechanisms. 

The electrochemical behavior of nitrites is schematically shown on figure Q-18-1, 
diagram C. They are typically anodic inhibitors thus capable of affecting the kinetics 
of anodic reactions. The inhibitors with highest "generality" of protection are the ones 




CATHODIC INHIBITOR BOTH ANODIC AND 

CATHODIC INHIBITOR 



Figure Q-18-1. Schematic showing the electrochemical behavior of nitrites. 



224 



that can change kinetics of both electrochemical reactions cathodic and anodic. Some 
esters of chromic acid and substituted carboxylate salts of heterocyclic amines have been 
demonstrated to fall in this category (fig. Q-18-1 , diagram C). A good source of references 
on VPI's are the Proceedings of a Symposium on Volatile Corrosion Inhibitors held in Houston 
in March of 1976, under the Sponsorship of the National Association of Corrosion Engineers 
(2400 West Loop South, Houston, Texas 77027). 

In summary of this discussion, I would like to mention that volatile corrosion inhibitors 
could indeed be a useful tool in the fight against atmospheric corrosion. However, informa- 
tion on them is scarce and scattered throughout technical literature and many questions 
regarding toxicity, effectiveness vs atmospheric pollutants, durability of protection, 
etc., still remain to be answered. 

B. M. Organ: It is clearly important in museum practice to have something that works with 
both ferrous and non-ferrous metals. We have avoided using Vapor Phase Inhibitors for 
many years because we could never guarantee that a storage space would always contain only 
ferrous metals. Curators keep objects made of various metals together. 

D. V. Pieahota: Is it true that some of the organic corrosion inhibitors, like benzotriazole, 
deteriorate upon exposure to UV? 

L. Fitzgerald: With regard to adverse effects of inhibitors, UV degradation of benzotriazole 
hasn't been a problem. Incralac^ samples on exposure 8 to 10 years show that the inhibitor 
is exhausted, but this is believed to be due to migration and volatilization of the inhibitor. 



Question 19: Under what conditions can corrosion inhibitors become corrosion stimulators? 



P. D. Weil: I wish to call attention to two important factors regarding protective coatings 
for outdoor metals: (1) the structure of the coating and (2) the necessity of a corrosion 
inhibitor in the coating. Brushed-on coatings, especially without inhibitor, can cause 
greatly accelerated corrosion, especially in the areas of the striation left by the brush 
hair in the coating. The coating will deteriorate in these areas first, and corrosion 
will be promoted by differential aeration in these hair-line fissures. This phenomenon 
has been observed to cause damage on a plated area of an outdoor sculpture. Protective 
coatings on outdoor sculpture should be applied by spray and in several thin coats. See 
P. Weil, The Approximate Two- Year Lifetime of Incralac on Outdoor Sculpture, Venice ICOM 
Conference Papers, October 1975. 

J. Kruger: Unless you use enough inhibitor, the inhibitor will stimulate corrosion at the 
active sites not covered. Unless you use the inhibitor correctly and adequately, it is 
best not to use any at all. 

R. M. Organ: You must have an enormous reserve of inhibitor to take care of local pockets 
of chloride. You shouldn't use inhibitors unless you know what you're doing. 

D. V. Pieahota: Can these organics such as BTA be broken down by ultraviolet light? 

L. D. Fitzgerald: Outdoor exposure tests on Incralac (with BTA) up to 10 years have shown 
no real serious breakdown. There is some volatilization of inhibitor, so that after 5 to 
6 years the inhibitor is pretty well deteriorated. 

W. T. Chase: What is the life of Incralac? 

L. D. Fitzgerald: 1 mil gives a life of 5 years, longer (10 years or so) with touch-ups. 
W, T. Chase: Indoors? 



^A lacquer developed by the International Copper Research Association for the outdoor pro- 
tection of copper alloys. 



225 



L. D. Fitzgerald: No data indoors, but it should last indefinitely indoors. 
W. T. Chase: What about holidays? 

L. D. Fitzgerald: Catch and repair. Mixed corrosion inhibitors are safe since they do 
not accelerate corrosion when present in insufficient concentration. The general limitation 
of inhibitors presently in use is in their anodic nature. In our investigations we succeeded 
in synthesizing compounds capable of producing protective films on both cathodic and 
anodic areas. 



Question 20: What is a simple, cheap, lasting method for preventing further corrosion? 
(No discussion) 



Question 21: What is the state of the art of protective coatings for metal sculptures 
displayed out of doors? (No discussion) 



Question 22: What is the present state of the art of protective coatings, their application, 
durability and reversibility for indoor or outdoor use on bronze, lead, silver, 
etc.? 



L. D. Fitzgerald: The development of INCRALAC, an acrylic finish for copper metals, was 
discussed in detail, with accompanying slides. This included criteria leading to (1) 
polymer selection, (2) inhibitor selection, and (3) solvent system. INCRA also examined 
variations in cleaning techniques. Acrylic polymers were found to be the best for main- 
tenance finishes since they were insensitive to ultraviolet and could be readily stripped. 
These when formulated with benzotrizole and a non-oxygen containing solvent such as toluene 
or xylene provided 5 to 10 years exterior durability. A minimum of 1 mil of dry film is 
required. Other coating systems which have been examined and show promise are (2) aliphatic 
polyurethanes which provide very hard surfaces for resistance to vandalism but are difficult 
to strip, (b) polyvinyl fluoride (Tedlar) finishes which must be applied on sheet metal in 
the plant. These can provide over 15 years of exterior durability. The cleaning and 
refinishing of the Mexico City Sports Palace and the Zambian National Assembly Building 
were discussed. Cleaners such as Turco-WO-1 and 5 percent sulfuric acid were used for 
these jobs. The rinse water contained 1 percent sodium carbonate. The Sports Palace, 
cleaned with Oakite-WO-1, needed no polishing and was lacquered with 25 percent solid 
INCRALAC (2 coats). The National Assembly Building, after cleaning with sulfuric acid, 
required polishing with rouge and prior to application of two coats of INCRALAC. Extended 
exposure tests at 12 outdoor sites throughout the U.S. have shown INCRALAC to be a durable 
finish for copper metals over periods up to 10 years. For indoor work when you're not in 
a hurry for drying, to avoid the orange peel surface with traditional INCRALAC, you can 
add a little MEK and you get a smooth coating which is great for indoor work. We've 
looked at the whole gamut of resin types, and we think INCRALAC is best for your purposes, 
for it is removable, and the other competitive coatings are hard to strip. 



Question 23: What nondestructive tests are suitable for field identification of metal 
objects? 



K. Morris: We have used an ultrasonic metal thickness gauge to give the thickness of 
metal objects. It is a sonar-like device, small enough to hold in your hand. This thick- 
ness measurement enables identification of various metal parts of a whole object for later 
reference. It can be particularly helpful to conservators of outdoor bronzes. 



226 



W. T. Chase: The use of a small, home-built metal detector may be helpful in ascertaining 
whether a much-corroded metal artifact has any metal remaining or not. Small metal detectors 
are commercially available for $5.00 up. The detector coil should be replaced with a small 
home/wound coil about the diameter of a pencil; the coil should be wound so that the 1-c 
or other circuit of the metal detector can be made to resonate. We have found a number of 
uses for one of these devices in our conservation laboratory. 

M. Goodway: Does anyone have suggestions about telling one metal from another in the 
field? 

F. Kalahari: Henry Hodges always said that there were only two methods: one was the 
magnet; the other, if you thought it might be lead, was to wipe it on paper. You will get 
a grey mark, if it's lead. Generally, in the field, you don't have electricity, you don't 
have any equipment or facilities for testing. Simple methods like that may be the only 
ones you can rely on. 

W. Goodway: (to W. Trousdale) Bill, didn't you have a case where the geologist hefted a 
piece, and from a knowledge of specific gravities and weight to be expected from volume, 
make a shrewder guess? 

W. Trousdale: That was me. (Laughter)... 

R. M. Organ: May I add one more? If you rub the clean metal object with your hands, and 
then sniff your hands, there are characteristic odors to the various metals which you get 
used to quite quickly. You can only do it if you have some means of wiping the hands 
clean before touching the object. Iron, copper, all these metals have characteristic 
odors. It might not work for you, but it does for me. 

M. Gooduay: In other words, use all your senses. 



Question 24: What methods of cleaning and stabilization of metals are suitable for 
employment at the archaeological site? (No discussion) 



Question 25: What practices should be avoided to prevent stress-corrosion cracking? 

B. F. Brown: You can't usually do anything about the stress, except assume that it's very 
high. If it's a copper object, restrain yourself from using ammoniacal cleaning material. 

S. Odell: Two or three examples of highly worked brass trumpet and horn bells which are 
badly cracked have come to my attention. Although the cracking in these cases has not 
been confirmed metal lographical ly as stress-corrosion cracking, it might be wise to avoid 
cleaning such objects with ammonia-containing solutions and polishes, such as Brasso. 

M. Goodway: If you can smell the ammonia, it's too much! 

S. Odell: Right. 



Question 26: What precautions need to be taken in the use of metal fastenings for displaying 
objects? (No discussion) 



Question 27: What sort of corrosion problems arise in the display of metal objects? 



227 



R. M. Organ: We have the usual problems that dissimilar metals should not be in contact. 
Sometimes display people like to fix metal plates onto vertical surfaces, and they use 
little metal hooks. This is fine, but the metal hooks may be of a different metal, and we 
get things happening at the point of contact. Another difficulty is that variations in 
temperature cause abrasion at those points of contact. Yet another is that when the 
display person drops it into position, he sometimes doesn't manage to drop it in gently, 
and it makes a dent on the edge of the object. 

To avoid all these problems, what we normally recommend is that each little metal 
hook be covered with a protective plastic sleeve. This creates another problem. In one 
place where I encountered this, the brass hook inside the plastic sleeve (probably made of 
plasticized polyvinyl chloride) which looked satisfactorily yellow and brassy before, had 
turned black. This is because of corrosion of the brass in contact with the polyvinyl 
chloride. 

What we recommend nowadays is to use the shrinkable sleeves that are sold by the 
electronics people. Out of the bundle of sleeves that you buy from Radio Shack or Lafayette, 
for instance, you select one that just fits on. You warm it up and it shrinks tightly 
into position. Then you have a permanent fixture. I think that these sleeves are made of 
polypropylene, which is a relatively safe material. 



Question 28: What are the conditions which can produce whisker disfiguration? 



T. Weisser: I am not certain what the person who wrote this question means by "whisker 
disfiguration." Does he mean whiskers of corrosion, single crystals, or what? 

We do get things which one could call whiskers growing on silver objects. We have 
had rather strange conditions at our museum which have produced silver sulfide crystals, 
not just tarnish. Long black whiskers first appeared on gold pieces, or pieces of jewelry 
which looked as if they were all gold. The whiskers seen on the metal parts of this 
jewelry turned out to be silver sulfide; the silver came from impurities of silver in the 
gold, from silver solder used in assembling the jewelry, or from silver wire used to 
attach gems. Why the crystals formed in a whiskery habit, I don't know. Tom Chase may 
have something to say about this, since he examined them at one point. I doubt whether 
they would truly be called whiskers. 

W. T. Chase: I think that the whisker-like crystals on the silver and the brown-to-black 
mossy-looking substance which forms on some bronzes in the Walters Collection (the renowned 
Walters Brown Fuzzies) are somehow connected, but I'm not sure just how. 

M. Goodway: Growth conditions. 

W. T. Chase: Yes, it must be. I think that the whiskers on the silver objects and on the 
gold objects with silver solder or silver wire came simply from the fact that they simply 
were not handled for long periods. As far as we could tell, some of that material was in 
the cases for 30 to 35 years, without ever coming out or being brushed or cleaned or 
anything. I wonder if some of the rest of us left things in cases untouched for that long 
whether we might not get the same effect? 

T. Weisser: I have noticed that whiskers have appeared on pieces of steel armor where 
there is silver inlay. I don't know whether or not these had not been handled for a long 
time. 

I also found them on a silver bust of Mr. Walters on the areas closest to the floor 
of the storeroom. One thing that I discovered in checking the conditions in the storeroom 
was that there were rubber mats on the floors. The mats had been put down to prevent 
breakage from dropping of objects. When you say that the "brown fuzzies" are related to 
the silver whiskers, I think that may be one relationship right there. They might both 
have been caused by the breakdown of these rubber mats that were put down in the 1930's. 
A lot of sulfur was being given off by this breakdown, and this might have been enough to 



228 



cause the silver sulfide crystals and the brown fuzzies (which do seem to have a lot of 
sulfur mixed in with them). 

M. Goodway: These whiskers also appear on bronzes, is that right? 

T. Weisser: The "brown fuzzies", a mossy-looking brown product, are on the bronzes, and 
the silver sulfide crystals are on the silver and gold pieces, and on anything that has 
silver attached to it in any way. 

M. Gooduay: I saw something that looked suspiciously like brown fuzzies on Chinese 
bronzes in the Art Institute of Chicago, in the exhibition cases. I don't think it's 
unique to the Walters. 

T. Weisser: We've had it on all the bronzes: Chinese bronzes, ancient bronzes. Renaissance 
bronzes, 19th century bronzes, almost every type. 

M. Goodway: I think it has to do with the enclosure, the stability, the time and so 
forth, but not necessarily that the enclosure be the Walters Art Gallery! 

T. Weisser: I'm very glad to hear that! 

K. Holm: We have experienced the growth of silver sulfide crystals on silver objects 
which have been held in place in the showcase by plasticine, which contains a fair amount 
of sulfur. 

M. Goodway: We still don't know what the conditions are for producing whisker disfiguration 
but I'm sure we'll all go home and look for them. 



Question 29: What are the characteristics of earlier conservation treatments that might lead 
to scholarly confusion? 



M. Goodway: Cyril Smith has one very good example of scholarly confusion. 

C. S. Smith: Figure Q-29-1 illustrates this. It is a photomicrograph showing the structure 
of an iron fibula from Slovenia, La Tgne period, aa. second century B.C. It is in the 
Mecklenberg collection at the Peabody Museum at Harvard University, and was one on the 
objects treated in hydrogen several decades ago by Willard M. Bright who proudly described 
his technique in the Museums Journal, 46, 1-5 (1946); also Mouseion, 55-56 (1946) 57-53. 
The fibula was made of tapered wire, about 2 to 4 mm diameter, beautifully coiled in the 
middle to form a helical spring. The microstructure consists of ferrite grains with 
particles of cementite (FesC) in clusters that retain the outlines appropriate to the 
pearlite constituent in a steel containing about 0.3 percent carbon after cooling in air 
from a temperature not much over 750 °C. However, the cementite particles are not in the 
lamellar form one would expect of such pearlite. The heating in hydrogen has caused it to 
spheroidize and the boundaries between the ferrite grains pass through the center of these 
patches in a most unusual way. One can reach no firm conclusions regarding the structure 
on the metal in the fibula when it left the hand of its maker. One can only guess that 
the metal had been worked hot, had not been quenched, and that it may have been slightly 
cold-worked in the finishing stages--all points that could have been read unequivocally 
from the microstructure of the object before heating. Moreover, the hydrogen treatment 
seems to have removed iron oxide slag inclusions. The metal is unusually clean. Was it 
so originally? 

No museum conservation laboratory would scrape all the writing off an ancient manuscript 
in order to have a clean piece of parchment to display, but, at least to someone concerned 
with technical history, the heating of metal artifacts is equally destructive of useful 
records of man's past knowledge and actions. 



229 




Figure Q-29-1 . Microstructure of iron fibula, La T§ne period, 
showing unusual structural details resulting from heating 
in hydrogen. Longitudinal section X500. 



M. Goodway: There is one point that I would like to stress in regards to the example that 
Cyril has shown. When you put an iron object in the furnace for that long at that tempera- 
ture, it shows signs of being at a controlled high temperature; what this tells the metallur- 
gist is that there was a thermocouple present. Thermocouples have not been with us for even 
as long as a hundred years. To find that sort of evidence tends to make one believe that 
there has been fakery involved. When this is in an object that also shows microstructures 
that are okay for an antiquity it can be extremely puzzling until one knows what particular 
treatment this sample has been through. Does anyone else have examples of things of this 
sort that they have run into? 

W. T. Chase: We run into this problem all the time. One of my favorite examples is in 
the Freer bronze book. {The Freer Chinese Bronzes, Vol. I, Catalogue, John A. Pope, 
Rutherford John Gettens, James Cahill, and Noel Barnard; Vol. II, Teohnieal Studies, 
Rutherford John Gettens; D.C., 1967 and 1969. The bronze referred to here is FGA 24.12, 
catalogue number 99. It appears in Vol. I on page 508 and Vol. II on page 192.) It is a 
late Chou Jiu of ovoid, necked form, dating from aa. 450 B.C. There is a large area on one 
side that is absolutely devoid of corrosion. This area looks as if it was made yesterday. 
The rest of the bronze carries the normal corrosion products. The naked area was painted 
over, and not discernible at the time of purchase. One day Mr. Gettens got the piece out 
for examination, and he saw a large area of fluorescence under ultra-violet light. He 
took some solvent to it, and found underneath it nice, uncorroded bronze. The area he 
revealed looks dead new. It isn't; this area simply did not corrode. 

In the Chinese Exhibition which came to Washington {The Chinese Exhibition, An Illustrated 
Handlist of the Exhibition of Archaeological Finds Of the People's Republic of China, 
National Gallery of Art, Washington, 1975; bronze rectangular hu number 117) there was 
another hiu with an area like this on the side of the top of the vessel. This vessel was 
excavated from a known context, and had not been repaired. 

In our examination of Chinese bronzes, we have seen a lot of repaint, repairs, etc. 
We've had people sample from repaired areas and publish the wrong composition. You constantly 
have to be on the lookout for earlier treatments. 



230 



Question 30: What methods are available for removing tarnish from metallic threads in 
texti les? 



M. Gooduay: Now there's a real problem. Has anybody got an answer besides the bristle 
brush? 

R. M. Organ: The most recent method for cleaning textile threads is to do it while you 
are washing the textile. You simply make sure that your textile is in contact with an 
aluminum sheet or screen. If necessary, you sew it down so that the metal thread is in 
contact with the aluminum wire mesh. Wash it in your usual detergent, which may be Orvus 
and is undoubtedly alkaline; when the washing is finished, the silver thread is also 
clean. It is an electrochemical reduction process. I think it was Joe Columbus who 
noticed this for the first time at the National Gallery, and it is he who should be telling 
us about it. 

We did some tests when we had to deal with a knife box which had silver braid decoration 
which was all black. We started out with the treatment intending to use this process. 
The process fails, of course, if you don't get electrical contact. We made tests of 
electrical conductivity along the length of the braid with a volt-ohm meter to find out 
how far apart we would have to put our stitches. The braid was broken in some places. We 
found that usually we had at least a one-centimeter distance which was electrically conductive 
We stitched at those intervals, and it came out beautifully. 

J. Columbus: This is an early sixteenth-century tapestry, about two and one-half by three 
and one-half meters in size, which we washed. First it was soaked overnight in a 10 
percent glycerin solution. The next day it was washed with Orvus. We had it on an aluminum 
screen, which the washing tank contains for support of the tapestry, but we didn't get the 
expected results from the aluminum. I think this is chiefly because of the way the silver 
is woven into the tapestry; it just can't possibly touch all of the surfaces. I then 
looked for some other method. 

In treating textiles with silver in them I have used silver dip. (Gold won't tarnish. 
For gold, it is just a matter of washing it with detergent.) Silver dip is a very acid, 
strong solution, but it is also very effective. If it is used fast enough, it can be 
safely used on a textile. For something as large as a tapestry the amount of time that it 
would have to remain in the solution might be excessive, and it might cause damage. The 
silver dip must be washed out with a detergent, quickly. 

Figures Q-30-1 and Q-30-2 show one small area in the tapestry before and after brushing 
with a plastic brush. You can see the difference. The small plastic brush measures about 
1/8 inch and about 1/4 inch wide. The ends of the bristle are very soft in feel to your 
finger. It is used with a rotary motion. It takes the corrosion off the surface, but 
there are many areas covered with adherent tarnish which the brush will not touch. There 
are many other areas where the weave is not sufficiently strong to even use the brush on 
it. Some non-mechanical method would have to be used. 

Almost a third of the total area of the tapestry is woven with silver. A fragment of 
the thread was examined at the Carnegie-Mellon Institute, and they told us that the major 
component was silver; there were also trace elements of copper, gold, lead, and magnesium, 
of less than 0.1 percent. It was a fairly pure form of silver. Why it's tarnished on 
some areas and not others I don't know. The yarns here are wool and silk. 

My question is, what are the hazards of using an electrolytic reduction method for 
removing tarnish from the silver thread? 

M. Goodway: Do we have any answers to this? What about the use of possibly alkaline 
solutions on a protein fiber? 

R. M. Organ: If you're prepared to wash it, then you can do it automatically during the 
washing process. If you're not prepared to wash it, electrolytic methods cannot be used 
at all . 



231 




WWA^Ws^ II lllltllltlJlHil'l 



Figure Q-30-1. A detail of a sixteenth century tapestry with silver thread 
before cleaning with a soft plastic brush. 



232 




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Figure Q-30-2. The same detail, after cleaning. 



233 



E. McMillan: A mild abrasive technique is the use of a plastic eraser which comes in two 
forms, a pencil shape and a rectangular form. It is called "Magic Rub"; I believe that 
the manufacturer is Faber/Castel 1 . Obviously one must be able to clean off the eraser 
shavings, which may be done with gentle vacuum cleaning, air blower, or soft brush. This 
technique has been used in our laboratory on the metallic braid decorating saddles, for 
example. Chemically, this material is a vinyl chloride polymer heavily plasticized with 
phthalate plasticizer and heavily filled with (calcium) carbonate. We have not observed any 
ill effects from its use. 



Question 31: What are the problems of the methods for consolidati ve reduction of objects 
of silver and lead? (No discussion) 



Question 32: What are the recommended procedures for the removal or the stabilization of 
corrosion products found on tin-lead-antimony alloys, especially britannia 
metal and pewter? What techniques of examination are necessary to distinguish 
between active and inactive corrosion on these metals? (No discussion) 



Question 33: How does one produce an initial protective film on bare metal during treatment? 

J. Krugev. One possibility for iron and iron alloys is this: after producing the bare 
metal by some sort of treatment, one should then, by electrochemical means, bring the 
metal to a potential where a passive film will form on the surface. This can be done by 
the use of a potentiostatic device, which allows you to select the potential to which you 
wish to bring the metal surface. You should bring the potential into the "passive" region. 
At these potentials, a passive film will form on the metal surface. 

The metal should be left in the passive region for as long as possible. This will 
make a good start at forming a protective film on the surface. It is anodization, actually, 
the production of anodic films. In the case of aluminum, these films are quite protective. 
They're not that protective for steel, and although there have been some attempts to form 
anodic films on copper, it is more tricky to do that. 

It is also important to remember, in anodic film formation, to use a buffered electrolyte 
at near neutral pH values. 



Question 34: What are the approaches in use by corrosion scientists which are applicable 
to the examination of metal objects? 

J. Kruger: I suggest the use of constant potential techniques as an alternative to the 
constant current ones mentioned in this meeting earlier for electrolytic treatments. 
These devices can be constructed cheaply from operational amplifiers available in the 
market. References are given in my paper (p. 59). 

M. Pourbaix: 1 remind you of what was said yesterday concerning the reduction of crusts 
and corrosion products and also of the very nice experiment which was shown by Jerry, on 
the work of Ulick Evans. I would like to suggest the following, which is very similar to 
methods being used in corrosion technology and corrosion science (see fig. Q-34-1). 

If one has to look to an object with corrosion products on it, one could study and 
also treat it by a carefully-controlled cathodic treatment which would act on reduction. 
Having here (a) a calomel electrode and an anode (b) of platinum which may both be moved 
together on the surface of the object, and here a source of direct current (c), measurement 



234 



calomel reference electrode 





mobile holder 



sipbon fitted witb a nylon 
brusb, and eventually \ 
witb a glass electrode 



platinum anode 



potentiostat 



specimen under test 
(bumid or immersed 
in a solution) \ 



SI 



Figure Q-34-1 . Device for potentiostatic reduction of specimens. 



of the current which is passing id], and a potentiostat (e) using the specimen as a cathode, 
it would be possible to impose, during treatment or in tests, a given electrode potential 
during a given period of time. I am not speaking here about the overall potential difference 
between anode and cathode, but about the potential difference between the cathode and the 
calomel reference electrode. It is now possible, with instruments which exist here at the 
Bureau, and which are gradually becoming very cheap, to make an experiment looking at the 
whole surface at given electrode potentials; this means that, if you believe in the electrode 
potential-pH diagrams, you may stay a given moment at a given point on the diagrams, and 
be sure that you are acting only on a given substance: for instance, only on the paratacamite, 
only on the cuprous chloride, only on the cuprous oxide, etc. For the examination of 
objects this might also be helpful. This approach is valid for every compound; you may do 
this on copper, on tin, on lead, on zinc, etc. Needless to say, the case of zinc is not 
very important, as zinc is always being dissolved. 

When you wish to treat an object by reduction, it may be of interest to know that you 
have no hydrogen evolved, or, alternatively, that you do have hydrogen. Having here (a) a 
calomel electrode, which might be made with a plastic capillary, or included in a toothbrush 
which was used to apply this solution, or combined with a glass electrode with which you 
can measure the pH, these quantities may be controlled. In addition to the possibilities 
for analysis of metal and corrosion products, this should assist in controlling the treat- 
ment. You may have a selective treatment, you may start with a high electrode potential 
and then go down. This is being used not only in corrosion studies, but also in analytical 
chemistry, for instance. I don't know if this has ever been done. I would like to suggest 
that this be done for looking into the composition of corrosion products and also for the 
treatment, which may be of greater interest to you. 

If you impose on a part of a specimen successively given decreasing values of electrode 
potential, you may proceed for instance, along line 1-4 in the copper-chlorine diagram given 
in figure Q-34-2. On point I the green paratacamite 3 (CuOH)2"Cu'Cl2) is stable; on point 2 
it may be reduced in a red cuprite CU2O and in 3 in metallic copper Cu; in 4 this reduction 
to copper may occur together with evolution of gaseous hydrogen. The intensities of current 
indicated on the meter (d) of figure Q-34-1 would give an idea of the quantities of chemical 
species taking part to the different stages of the reduction. 



235 




Figure Q-34-2. Potential-pH equi- 
librium diagram for the ternary 
system Cu-Cl-HzO at 25 °C 
(355 ppm CI ) . 



Figure Q-34-3 relates to the tin-water system. On point I you are in the region of 

passivation by SnOa (cassiterite) . On point 2 this oxide may be dissolved and on point Z 

metallic tin may be formed, which will lead on point 4 to the formation of unstable gaseous 
tin hydride SnHi^. Below line a gaseous hydrogen may be evolved. 



According to figure Q-34-4, which relates to lead, due to the high solubility of PbO, 
the metal is dissolved on point I if no CO2 is present; if there is CO2 > the white lead 
carbonate PbCOs (cerussite) is stable. On point 2 metallic lead may be formed, leading on 
point Z to the formation of unstable gaseous lead hydride PbH2. 



Figure Q-34-5 shows that zinc will most of the time be leached out from corrosion 
products, due the high solubility of its lower oxides, carbonates, etc. By putting a 
piece of metallic zinc in contact with corroded bronze in a solution corrosive to zinc, 
according to the method suggested by U. R. Evans, and shown in figure Q-12-1 (p. 218), 
the electrode potential at the contact between bronze and zinc will be in the neighborhood 
of point I (about -800 mV or -1050 mV). As shown by figures Q-34-2 through 4, this may 
reduce the corrosion products of copper, tin and lead with deposition of the corresponding 
metals. 



Concerning silver, a diagram relating to the system silver-sulphur-water, established 
by Horvath, which I do not have here with me, may be helpful for predetermining the conditions 
where the black silver sulfide may be reduced to metallic silver. 



236 




(a) 



(b) 



Figure Q-34-3. (a) potential -pH 
equilibrium for the system tin- 
water at 25 °C. (b) theoretical 
conditions of corrosion, immunity, 
and passivation of tin at 25 °C. 

2 0 2 4 6 8 10 12 14 16 

pH 




237 




pH pH 

(a) (b) 



Figure Q-34-4. Potential-pH equilibrium diagram for the system 
lead-water at 25 °C (top). Theoretical conditions of corrosion 
immunity, and passivation of lead (at 25 °C); (a) for solutions 
free from CO2 ; and (b) for solutions containing CO2 (1 g-mol/1) 



238 





Figure Q-34-5. Potential -pH equilibrium diagram for the system 
zinc-water at 25 °C (top). Theoretical conditions of corrosion, 
immunity, and passivation of zinc (at 25 °C); (a) for solutions 
free from CO2; and (b) for solutions containing CO2 (1 g-mol/1). 



239 



Question 35: What sort of research would conservators like to see corrosion scientists do? 



W. T. Chase: I guess the answer to that is really "a lot." One of the problems that I've 
been very interested in over the last few years is the treatment of bronze disease with 
benzotriazole. We talked about the criteria for its effectiveness yesterday. It seems to 
work, and it seems to stop what looks like ongoing corrosion. At least it stops the 
bright green stuff from coming out. But the question is, why does it work? Where is it 
going in the structure of these corroded bronzes? I don't think that at this point anybody 
knows. Finding out the answers is a project that we would really like to see come corrosion 
scientist or someone who has access to instruments which would be helpful to pick up and 
run with. 

M. Pourbaix: I have learned this morning that some work has been done using INCRAlac, 
which is made with benzotriazole, on gold. This is also where more research may improve 
the situation. The color of the structures doesn't seem to be the color of gold. 

B. M. Organ: A research that I would like to see done is one directed to discovering the 
conditions under which bronzes are attached and eaten away invisibly in the presence of 
cuprous chloride. The sort of experiment that might yield the answers is to set up a 
system with cuprous chloride on the surface of metallic copper, and measure the corrosion 
potential under various partial pressures of oxygen. If there is not oxygen present, this 
might simulate the situation where we have, in a real bronze, cuprous chloride covered 
over with an impermeable crust of cuprous oxide. At the other extreme, with the partial 
pressure of oxygen as we find it in the atmosphere, this would simulate the condition 
where we have the more normal cuprous oxide cover, which is permeable. This experiment 
would give us a range of conditions, and would tell us whether and how fast the copper is 
being attacked. 

M. Goodway: You are suggesting that the cuprous chloride is, perhaps, not so much of a 
problem except when the oxygen is exhausted. 

R. M. Organ: Yes, quite. As far as we know, we can stop all this happening just by 
keeping things dry and free from oxygen. I'd like to know precisely what the conditions 
would have to be. Alternatively, it would be nice to know at what value of ambient relative 
humidity does the reaction stop. 

M. Gooduay: Are there some other questions, perhaps, of interest? Well, it seems to me 
that this is one of the most important questions on the list. In our thinking about the 
phenomena that we see on objects and in trying to formulate answers, it might be useful to 
formulate our questions more exactly, and try to marshal! our evidence very carefully, and 
talk to a corrosion expert. 

T. Weisser: Would it be possible to have a list of those corrosion scientists and other 
scientists who would be willing to do analysis for museums? Often museums do not have 
sophisticated equipment or time and funding to do this sort of work, and it would be a 
great help to know where to get analyses done. 



240 



CORROSION AND METAL ARTIFACTS 
March 17-19, 1976 



List of Attendees 



A 

Etiene Ahseneau 

Village Histohique Acadien 

P.O. Box 820 

Cahaquet N.B. EOBIKO 

Canada 

Carol Aiken 

45 First Street 

Waterford, NY 12188 

Will iam J. Andahazy 
Naval Ship Research & 
Development Center 
Bethesda, MD 20084 

Laurie A. Anderson 

16 Ransom Road, Apt. 14 

Brighton, MA 02135 

Walter Angst 

2602 Evans Drive 

Silver Spring, MD 20902 

Frederick B. Anthon 
4834 Rodmann St. , NW 
Washington, DC 20016 

Lynda Aussenberg 
Freer Gallery of Art 
Smithsonian Institution 
Washington, DC 20560 



B 

Lars Barkman 
Wasavarvet 

National Maritime Museum 
Stockholm, Sweden 

U. Bertocci 

National Bureau of Standards 
B254 Matls Bldg 
Washington, DC 20234 

C. Ernest Birchenall 
University of Delaware 
Newark, DE 19711 

Elena Borowski 
Office of Museum Programs 
Smithsonian Institution 
Washington, DC 20560 



F. E. Brinckman 
National Bureau of Standards 
A329 Matls Bldg 
Washington, DC 20234 

R. P. Brookshire 

Brooktronics Engineering Corp. 

13161 Sherman Way 

North Hollywood, CA 91605 

B. F. Brown 
American University 
Washington, DC 20016 

Herbert D. Bump 

Bureau of Historic Sites 

& Properties 
401 E. Gaines 
Tallahassee, FL 32304 

Martin Burke 
703 A Street, NE 
Washington, DC 20002 

Harry C. Burnett 
National Bureau of Standards 
B264 Matls Bldg 
Washington, DC 20234 

Ann M. Byrne 

Canadian Conservation Institute 
236 St. George Street 
P.O. Box 645 

Moncton, New Brunswick EIC 8M7 
Canada 



C 

Janice H. Carlson 
Winterthur Museum 
Winterthur, DE 19735 

Robert R. Carter 
Art Instttute of Ch_icago 
Michigan at the foot of Adams 
Chicago, IL 60603 



Henry Cohen 
Museum of Modern Art 
21 West 53 Street 
New York, NY 10019 

David L. Colglazier 
Old Sturbridge Village 
Conservation Laboratory 
Sturbridge, MA 01566 

Joseph V. Columbus 
National Gallery of Art 
6th & Constitution Ave., NW 
Washington, DC 20565 

Clifford Craine 
Fogg Art Museum 
Conservation Dept. 
Cambridge, MA 02138 

ICirkwood M. Cummtngham 
Smithsonian Institution 
Conservation-Analytical Lab 
Washington, DC 20560 



D 

Diane E. Dayies 
University of Pennsylvania 
University Museum 
33rd & Spruce Streets 
Philadelphia, PA 19174 



E 

Douglas G. Everstine 
Naval Ship Research 

& Development Center 
Bethesda, MD 20084 



F 

Inge Fiedler 

The Art Institute of Chicago 
Michigan at Adams Street 



W. T. Chase 
Freer Gallery of Art 
Smithsonian Institution 
Washington, DC 20560 

Seymour K. Coburn 
U.S. Steel Corp. 
600 Grant Street 
Pittsburgh, PA 15232 



Chicago, IL 60630 

Louis D. Fitzgerald 
ASARCO, Inc. 

Central Research Laboratory 
South Plainsfield, NJ 07080 



241 



1 



Elisabeth W. pitzHugh 
Freer Gallery of Art 
Smithsonian Institution 
Washington, DC 20560 

Elizabeth M. Flavin 
107 N. Van Pelt Street 
Philadelphia, PA 19103 

R. T. Foley 
American University 
Washington, DC 20016 

Carol W. Forsythe 
Winterthur Museum 
Winterthur, DE 19735 

Alan D. Franklin 
National Bureau of Standards 
A355 Matls Bldg 
Washington, DC 20234 

James M. Frantz 
Columbia University 
334 Riverside Drive 
New York, NY 10025 



G 

Mary Garvin 

2340 Generation Drive 

Reston, VA 22091 

Peter P. Gaspar 
Washington University 
Department of Chemistry 
St. Louis, MO 63130 

David C. Goist 
Intermuseum Laboratory 
Allen Art Bldg 
Oberlin, OH 44074 

Martha Goodway 
Smithsonian Institution 
Washington, DC 20560 

Virginia Greene 
University Museum 
33rd & Spruce Streets 
Philadelphia, PA 19174 

Earl A. Gulbransen 
University of Pittsburgh 
848 Benedum Hall 
Pittsburgh, PA 15261 



H 

Frances Halahan 
Departamento de Restauracion 
del Patrimonto Cultural 



Ex-Convento de Churubusco 
Mexico 21 DF 

Barbara Hall 
University of Chicago 
Oriental Institute Museum 
1155 East 58th Street 
Chicago, IL 60637 

Kurt F. J. Heinrich 
National Bureau of Standards 
A121 Chem Bldg 
Washington, DC 20234 

Don B. Heller 
Winterthur Museum 
Winterthur, DE 19735 

Donald C. Hemming 
Revere Copper & Brass, Inc. 
Research & Development Center 
Rome, NY 13440 

Charles E. S. Hett 
National Historic Parks 

& Sites 
Ottawa, Ontario 
Canada 

J. D. Hoffman 

National Bureau of Standards 
B364 Matls Bldg 
Washington, DC 20234 

Frederick L. Hollendonner 
Cleveland Museum of Art 
11150 East Blvd. 
Cleveland, OH 44106 

Knud E. Holm 

The National Museum of Denmark 

Nationalmuseet 

Brede, 2800 Lyngby 

Denmark 

Evie Z. Holmberg 
Fogg Art Museum 
Broadway & Quincy St. 
Cambridge, MA 02138 

Bruce R. Hood 
Naval Ship Research 

& Development Center 
Bethesda, MD 20084 

Walter R. Hopwood 
Smithsonian Institution 
Conservation-Analytical Lab 
Washington, DC 20560 

Nikki Horton 
Smithsonian Institution 
Conservation-Analytical Lab 
Washington, DC 20560 



Patricia Houl ihan 
Museum of Modern Art 
21 West 53rd Street 
New York, NY 10019 

John D. Howell 
Atlantic Richfield Co. 
400 E. Sibley Blvd. 
Harvey, IL 60426 

Rodan D. House 
Southside Historical Sites 
Dept. of Anthropology 
College of William & Mary 
Williamsburg, VA 23185 



J 

Carol J. Jack 
Royal Ontario Museum 
100 Queen's Park 
Toronto, Ontario 
Canada M5S 2C6 

Victoria Jenssen 
University Museum 
427 South 45th Street 
New York, NY 19104 

Deborah F. Jewett 

Atlantic Conservation Center 

National Museums of Canada 

P.O. Box 645 

Moncton, New Brunswick 

Canada 

Si an Jones 

Walters Art Gallery 

600 North Charles Street 

Baltimore, MD 21201 



IC 

W. Boulton Kelly 
Architectural Conservators Ltd. 
6229 N. Charles Street 
Baltimore, MD 21212 

Youngja Kim 
Museum of Modern Art 
21 West 53rd Street 
New York, NY 10019 

George H. Kordela 
National Museum of Man 
Ottawa, Ontario KIA 0M8 
Canada 

Jeanne L. Kostich 
The Metropolitan Museum 
15 West 54 Street 
New York, NY 10019 



242 



Barbara S. Kroll 
Fogg Art Museum 
Harvard University 
Cambridge, MA 02138 

Jerome Kruger 

National Bureau of Standards 
B254 Matls Bldg 
Washington, DC 20234 

Koel Kunz 

New York University 
Conservation Center 
1 East 78th Street 
New York, NY 10021 



L 

Bernard Leech 

Royal Ontario Museum 

100 Queens Park 

Toronto, Ontario M5S 2C6 

Canada 

Geoffrey M. Lenmer 
The Baltimore Museum of Art 
Art Museum Drive 
Baltimore, MD 21218 

Steven A, Leon 

American Museum of Natural 

History 
1232 Madison Avenue 
New York, NY 10028 

Ray Lindberg 
Reynolds Metals Co. 
P.O. Box 27003 
Richmond, VA 23261 

Philip A. Lins 
Winterthur Museum 
Winterthur, DE 19375 

Margaret E. Loew 
Winterthur Museum 
Winterthur, DE 19735 



M 

Edward McManus 

4811 South 31st Street 

Apt. C-1 

Arlington, VA 22206 

Eleanor McMillan 
Conservation-Analytical Lab 
Smithsonian Institution 
Washington, DC 20560 



R. A. Meussner 
Naval Research Lab 
Washington, DC 20034 

Joan Mishara 
Smithsonian Institution 
Conservation-Analytical Lab 
Washington, DC 20560 

Gillian Moir 

Royal Ontario Museum 

100 Queen's Park 

Toronto, Ontario M5S 2C6 

Canada 

Barbara Miller 
Smithsonian Institution 
Conservation-Analytical Lab 
Washington, DC 20560 

Kenneth Morris 
Washington University 
Box 1105 

St. Louis, MO 63130 
J. L. Mullen 

National Bureau of Standards 
B254 Matls Bldg 
Washington, D.C. 20234 

Cleo Mullins 

1119 South 16th Street 

Arlington, VA 22202 



N 

Dale E. Newbury 
National Bureau of Standards 
A121 Chem Bldg 
Washington, DC 20234 

N. A. Nielson 
DuPont 

2 Walnut Ridge Road 
Wilmington, DE 19807 

Jane Norman 

Smithsonian Institution 
Anthropology-Conservation Lab 
Washington, DC 20560 



0 

Scott Odell 

Smithsonian Institution 
4123 MHT 

Washington, DC 20560 

Fielding Ogburn 
National Bureau of Standards 
B254 Matls Bldg 
Washington, DC 20234 



Jacqueline S. 01 in 
Smithsonian Institution 
Washington, DC 20560 

R. M. Organ 

Smithsonian Institution 
Washington, DC 20560 



P 

George Papadopulos 
Hispanic Society of America 
159 Watchung Ave. 
Montclair, NJ 07043 

Alexis Pencovic 

Asian Art Museum 

Golden Gate Park 

San Francisco, CA 94118 

Curtiss E. Peterson 
Bureau of Historic Sites 

& Properties 
401 East Gaines Street 
Tallahassee, FL 32301 

Dennis V. Piechota 

Peabody Museum of Archaeology 

and Ethnology 
11 Divinity Ave. 
Cambridge, MA 02138 

Marcel Pourbaix 
CEBELCOR 

Ave Paul Meger-Gate 2 
Brussels, Belgium 



R 

A. J. Raffalovich 

38 Queens Drive 

Little Silver, NJ 07739 

Shelley N. Reisman 
Winterthur Museum 
Winterthur, DE 19735 

Mervin Richard 
Intermuseum Laboratory 
Allen Art Bldg 
Oberlin, OH 44074 

W. D. Richey 
Chatham College 
Pittsburgh, PA 15232 

Dan Riss 

National Park Service 
Harpers Ferry, WV 25425 



243 



B. L. Rogers 
National Park Service 
Division of Museum Services 
Harpers Ferry, WV 25425 

Carolyn L. Rose 
Smithsonian Institution 
Museum of Natural History 
Washington, DC 20560 

Amy Rosenberg 
University of Michigan 
Kelsey Museum 
434 S. State St. 
Ann Arbor, MI 48109 

Marvin C. Ross 
Smithsonian Institution 
Washington, DC 20550 



S 

E. V. Sayre 
Museum of Fine Arts 
465 Huntington Ave. 
Boston, MA 02115 

Cryil S. Smith 
Massachusetts Institute 

of Technology 
Cambridge, MA 02139 

James B. Smith, Jr. 
National Park Service 
Division of Museum Services 
Harpers Ferry, WV 25425 

Jack Soul tanian , Jr. 
Conservation Center 
New York University 
1 East 78th Street 
New York, NY 10021 

Janet L. Stone 
National Park Service 
Harpers Ferry, WV 25425 

Shelley Sturman 
Harvard University 
Cambridge, MA 02139 



T 



Steven Tatti 
Hirshhorn Museum 

& Sculpture Garden 
8th and Independence Ave. 
Washington, DC 20003 

John M. Taylor 

Canadian Conservation Institute 
National Museums of Canada 
Ottawa, Ontario MIA 0M8 
Canada 

Kathleen K. Taylor 
Brookhaven National Lab 
Chemistry Dept. 
Upton, NY 11973 

William T. Tearman 
7226 Kidmore Lane 
Lanham, MD 20801 

William Trousdale 
Smithsonian Institution 
Washington, DC 20560 

Linda K Tucker 

Copperstown Graduate Program 
809 River Street 
Troy, NY 12180 



U 

G. Ugiansky 

National Bureau of Standards 
B254 Matls Bldg 
Washington, DC 20234 



V 

Jean Van Muylder 
University of Brussels 
Avenue F. Roosevelt, 50 
B-1050 Brussels, Belgium 

L. Van Zelst 

The Metropolitan Museum of Art 
5th Avenue at 82nd Street 
New York, NY 10028 



W 



Steven Weintraub 
The Metropolitan Museum of Art 
5th Avenue at 82nd Street 
New York, NY 10028 

Norman Weiss 

Preservation Technology Group, 
Ldt. 

2230 Q Street, NW 
Washington, DC 20008 

Terry D. Weisser 
The Walters Art Gallery 
600 N. Charles Street 
Baltimore, MD 21201 

Judith Weston 

Detroit Institute of Arts 

5200 Woodward 

Detroit, MI 48202 

Gayle Wever 

243 South Melville Street 
Philadelphia, PA 19139 

Florence E. Whitmore 
Museum of Fine Arts (Res. Lab) 
465 Huntington Avenue 
Boston, MA 02115 

John Winter 
Freer Gal lery of Art 
Smithsonian Institution 
Washington, DC 20560 

Sara J. Wolf 

Hampton Institute College 

Museum 
Hampton, VA 23668 



Z 

Thomas 0. Ziebold 
Braddock Services, Inc. 
17200 Longdraft Road 
Gaithersburg, MD 20760 



Phoebe D. Weil 
Christopher Tahk Washington University 

Cooperstown Graduate Program Box 1105 
Lake Road St. Louis, MO 63130 

Copperstown, NY 13326 



244 



NBS-1 14A (REV, 7-73) 


U.S. DEPT. OF COMM. 1 . PUB LIGATION OR RE PORT NO. 2. Gov't Accession 
BIBLIOGRAPHIC DATA No 

SHEET NBS SP-479 


3» Recipient s -Accession No. 


4. TITLE AND SUBTITLE 

Corrosion and Metal Artifacts 

A Dialogue Between Conservators and Archaeologists and 
Corrosion Scientists 


5» Publication Date 

July 1977 


6. Performing Organization Code 


7. AUTHOR(S)B. F. Brown, H. C. Burnett, W. T. Chase, 
M, Goodway, J. Kruger, and M. Pourbaix 


8. Performing Organ. Report No. 


9. PERFORMING (ORGANIZATION T^AME^ AND ADDRESS 

NATIONAL BUREAU OF STANDARDS 
DEPARTMENT OF COMMERCE 
WASHINGTON, D.C. 20234 


10. Project/Task/Work Unit No. 


11. Contract/Grant No. 


12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 

National Bureau o£ Standards, Washington, D.C, American 
University, Washington, D.C, Smithsonian Institution, 
Washington, D.C, Washington Conservation Guild, CEBELCOR, 
Brussels, Belgium 


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C ov e r e d 

Final 


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15. SUPPLEMENTARY NOTES 



Library of Congress Catalog Card Number: 77-608103 



16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant 
bibliography or literature survey, mention it here.) 

This book is the formal report of the proceedings of the seminar 
on Corrosion and Metal Artifacts. The volxame contains the tutorial 
lectures on the aspects of corrosion science and engineering of 
relevance to co nservators and archaeologists and, background lectures 
which are addressed to corrosionists with activities and problems 
in the conservation of metallic artistic artifacts. The report also 
c(pntains the full discussion (attendee) of the structured questions 
distributed before the meeting. The report is well documented with 
illustrations. 



17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper 

name; separafed by semico/ons; Archaeological f inds , preservation of; conservation 
of metal artifacts; corrosion, inhibiting of; corrosion, treatment 
methods; metal artifacts, restoration of; patina, natural; patina, 
artifically produced. 



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NBS TECHNICAL PUBLICATIONS 



PERIODICALS 

JOURNAL OF RESEARCH reports National Bureau 
of Standards research and development in physics, 
mathematics, and chemistry. It is published in two 
sections, available separately: 

• Physics and Chemistry (Section A) ^0 
Papers of interest primarily to scier* ^."^ orking in 
these fields. This section covers a br .ige of physi- 
cal and chemical research, wif- ^r emphasis on 
standards of physical measu*- ^ y* , fundamental con- 
stants, and properties of m' ^<<.* '.dsued six times a year. 
Annual subscription: D- ^^\o^c, $17.00; Foreign, $21.25. 

• Mathematical Sci^ ^(jp.^^vJ'ection B) 

Studies and com^- ^* .is designed mainly for the math- 
ematician and .Q'Ck^dtical physicist. Topics in mathemat- 
ical statis*^"^^c;e?^.ieory of experiment design, numerical 
analysis -xJP jretical physics and chemistry, logical de- 
sign ^ programming of computers and computer sys- 
jnort numerical tables. Issued quarterly. Annual 
ocription: Domestic, $9.00; Foreign, $11.25. 

DIMENSIONS/NBS (formerly Technical News Bulle- 
tin) — This monthly magazine is published to inform 
scientists, engineers, businessmen, industry, teachers, 
students, and consumers of the latest advances in 
science and technology, with primary emphasis on the 
work at NBS. The magazine highlights and reviews 
such issues as energy research, fire protection, building 
technology, metric conversion, pollution abatement, 
health and safety, and consumer product performance. 
In addition, it reports the results of Bureau programs 
in measurement standards and techniques, properties of 
matter and materials, engineering standards and serv- 
ices, instrumentation, and automatic data processing. 

Annual subscription: Domestic, $12.50; Foreign, $15.65. 

NONPERIODICALS 

Monographs — Major contributions to the technical liter- 
ature on various subjects related to the Bureau's scien- 
tific and technical activities. 

Handbooks — Recommended codes of engineering and 
industrial practice (including safety codes) developed 
in cooperation with interested industries, professional 
organizations, and regulatory bodies. 
Special Publications — Include proceedings of conferences 
sponsored by NBS, NBS annual reports, and other 
special publications appropriate to this grouping such 
as wall charts, pocket cards, and bibliographies. 
Applied Mathematics Series — Mathematical tables, man- 
uals, and studies of special interest to physicists, engi- 
neers, chemists, biologists, mathematicians, com- 
puter programmers, and others engaged in scientific 
and technical work. 

National Standard Reference Data Series — Provides 
quantitative data on the physical and chemical proper- 
ties of materials, compiled from the world's literature 
and critically evaluated. Developed under a world-wide 
program coordinated by NBS. Program under authority 
of National Standard Data Act (Public Law 90-396). 



NOTE: At present the principal publication outlet for 
these data is the Journal of Physical and Chemical 
Reference Data (JPCRD) published quarterly for NBS 
by the American Chemical Society (ACS) and the Amer- 
ican Institute of Physics (AIP). Subscriptions, reprints, 
and supplements available from ACS, 1155 Sixteenth 
St. N.W., Wash. D. C. 20056. 

Building Science Series — Disseminates technical infor- 
mation developed at the Bureau on building materials, 
components, systems, and whole structures. The series 
presents research results, test methods, and perform- 
ance criteria related to the structural and environmental 
functions and the durability and safety characteristics 
of building elements and systems. 

Technical Notes — Studies or reports which are complete 
in themselves but restrictive in their treatment of a 
subject. Analogous to monographs but not so compre- 
hensive in scope or definitive in treatment of the sub- 
ject area. Often serve as a vehicle for final reports of 
work performed at NBS under the sponsorship of other 
government agencies. 

Voluntary Product Standards — Developed under proce- 
dures published by the Department of Commerce in Part 
10, Title 15, of the Code of Federal Regulations. The 
purpose of the standards is to establish nationally rec- 
ognized requirements for products, and to provide all 
concerned interests with a basis for common under- 
standing of the characteristics of the products. NBS 
administers this program as a supplement to the activi- 
ties of the private sector standardizing organizations. 
Consumer Information Series — Practical information, 
based on NBS research and experience, covering areas 
of interest to the consumer. Easily understandable lang- 
uage and illustrations provide useful background knowl- 
edge for shopping in today's technological marketplace. 

Order above NBS publications from: Superintendent 
of Documents, Government Printing Office, Washington, 
D.C. 20J,02. 

Order following NBS publications— NBSlR's and FIPS 
from the National Technical Information Services, 
Springfield, Va. 22161. 

Federal Information Processing Standards Publications 
(FIPS PUBS) — Publications in this series collectively 
constitute the Federal Information Processing Stand- 
ards Register. Register serves as the official source of 
information in the Federal Government regarding stand- 
ards issued by NBS pursuant to the Federal Property 
and Administrative Services Act of 1949 as amended, 
Public Law 89-306 (79 Stat. 1127), and as implemented 
by Executive Order 11717 (38 FR 12315, dated May 11, 
1973) and Part 6 of Title 15 CFR (Code of Federal 
Regulations). 

NBS Interagency Reports (NBSIR) — A special series of 
interim or final reports on work performed by NBS for 
outside sponsors (both government and non-govern- 
ment). In general, initial distribution is handled by the 
sponsor; public distribution is by the National Techni- 
cal Information Services (Springfield, Va. 22161) in 
paper copy or microfiche form. 



BIBLIOGRAPHIC SUBSCRIPTION SERVICES 



The following current-awareness and literature-survey 
bibliographies are issued periodically by the Bureau: 
Cryogenic Data Center Current Awareness Service. A 

literature survey issued biweekly. Annual subscrip- 
tion: Domestic, S25.00 ; Foreign, $30.00'. 
Liquified Natural Gas. A literature survey issued quar- 
terly. Annual subscription: $20.00. 



Superconducting Devices and Materials. A literature 
survey issued quarterly. Annual subscription: $.30.00 . 
Send subscription orders and remittances for the pre- 
ceding bibliographic services to National Bureau of 
Standards, Cryogenic Data Center (275.02) Boulder, 
Colorado 80302. 



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